I've noticed this being one of the more common theme for OC Sentai, but other than Power Rangers based on Pokemon not a whole lot seems to go into these ideas. I for one got a rather neat backstory to go with it;Sectrobeast Pokemon Ranger
A rather strange epidemic has broken out, a literal plague of supernatural origin is starting to turn otherwise normal Pokemon into mutated humanoid abominations now classified as "Predacores", berserk beyond reason and insanely powerful. The only thing capable of stopping these beasts and reversing this disease's effects are the "SectroBeastors", transforming Pokemon Rangers who fuse with their "Primary Pokemon" through "Sectrobeast Binary-Armor Tech" gaining all the powers of the Pokemon along with a few unique cybernetic tools and weaponry to further augment their fighting capabilities, including the "Sectro-Drive" which grants each ranger a unique super ability. Another primary tool is the "Sectro Anti-Virus", the very tool most needed to cure a Predacore and return it to it's original Pokemon self, but in order to use it they must weaken the target Predacore seemingly near death when the virus is rendered at it's weakest. The "Primary Pokemon" can also become Zords for the SectroBeastors to pilot against the more colossal "situations". Ash Ketchum- the Yellow Ranger. He was just a civilian until he got a hold of a SectroBeast Navi, after his incredible sortie he was made a Pokemon Ranger and thus put on the SectroBeastor Team. He and his Primary Pokemon Pickachu possess such a strong bond that they were perfect for the Sectrobeast Binary-Armor Tech, in fact it is possible that because of their bond that they have the potental to be the strongest of the SectroBeastors. His Sectro-Drive is super sonic speed.
Solana- the Blue Ranger, her Primary Pokemon is Piplup. Because of her sorties she is one of the most famous of the Pokemon Rangers and is regarded as sort of an idol. Her Sectro-Drive is crystal armor.
Kellyn- the Red Ranger, his Primary Pokemon is Chimchar. He is declared the eleventh of twelve Rangers to become Top Rangers, he and Solana are good friends. At first he doesn't think too highly of Ash but is still able to work alongside him. His Sectro-Drive is Herculean strength.
Kate- the Green Ranger, her Primary Pokemon is Treecko. She is Kellyn's twin sister, despite not being a Top Ranger herself she is just as skilled and capable as he is. She has a sort of small crush on Ash. Her Sectro-Drive is healing energy.
Jack Walker- the White Ranger, his Primary Pokemon is Absol. The leader of the team and Ash's new senpai, he has high hopes for the new recruit and so far is impressed with his progress. As the leader Jack is quite the level headed fighter, able to be the glue that holds the team together when things go wrong. His Sectro-Drive is invisibility.
Commander Fox McCloud- His Primary Pokemon is Lugia. He is actually an anthropomorohic fox from the regions of space who came here after losing his entire team in battle against an evil empire, having defeat that empire he came to Earth hoping to help this planet in need however he can eventually becoming the Supreme Commander of the Pokemon Rangers.
As things heat up, two new SectroBeastors appear, though they possess the same Sectrobeast Binary Armor Tech the actual tech they possess is of mysterious origin just as they themselves are. It is believed that they hail from different timelines altogether; Link- the Lucario Ranger, his Primary Pokemon is Lucario. A mysterious swordmaster garbed in green who wields other-worldly magical items. He has a past history with the true fiends behind the epidemic. He has two Sectro-Drives, the first is light sword and the second is time flux.
Samus Aran- the Entei Ranger, she actally has three Primary Pokemon which it turns out are the three Legendary Beasts Entei, Raikou and Suicune. She claims to be a bounty hunter from space, sent here to destroy a creature known as "Mother Brain". She has two Sectro-Drives, the first is morph ball and the second is charge beam.
The Primary Pokemon aren't limited to just overall henshin and Zords, they also provide signature weapons. The SectroBeastors can also bring out additional Pokemon to form additional ranger weapons and (of course) Auxilery Zords. Pikachu- Aside from being Ash's henshin, he also gives Ash the Sectro Thunder Hammer that can configure between a small one hand weapon to a large two hand weapon. As a Zord he functions as a single wheeled motorcycle vehicle.
Bulbasaur- He is a secondary Pokemon used by Ash that gives any Ranger the Sectro Grass Lantern, a powerful flood light module that can pierce any form of darkness and can also fire off an intense solar energy laser. As an Auxilery Zord he can replace Pikachu as the head of the Palkia or Dialga Megazord and gives the Megazord the Radiance Crown attack which gathers solar energy in and unleashes it in a blinding wave rendering a target helpless.
Piplup- Aside from being Solara's henshin, she also gives Solana the Sectro Freeze Pistols that fire both water and ice ammunition. As a Zord she functions much like a snow-mobile with a drill beak.
Torchic- She is a secondary Pokemon used by Solana that gives any Ranger the Sectro Spinning Top, a tactical kinetic incinerary grenade that can be launched and remote guided with the gun-like launcher. As an Auxilery Zord she can replace Piplup as the left arm of the Palkia or Dialga Megazord and gives the Megazord the Phoenix Wing which is a powerful flame energy shield shaped like a bird's wing, it is inpenetrable.
Chimchar- Aside from being Kellyn's henshin, he also gives Kellyn the Sectro Burning Claws that are basically jackhammer powered gauntlets able to burn through solid steel. As a Zord he functions much like a primate robot.
Totodile- He is a secondary Pokemon used by Kellyn that gives any Ranger the Sectro Jaw Cannon which functions both as a clamping steel jaw weapon and a high-powered hydro grenade launcher. As an Auxilery Zord he can replace Chimchar as the right arm of the Palkia or Dialga Megazord and gives the Megazord the Steel Fang Fist attack that uses the clamping jaw teeth to tear through a target with a single punch.
Treecko- Aside from being Kate's henshin, he also gives Kate the Sectro Whiplash Lance which is a segmented staff that bears a powerful ionizer blade. As a Zord he functions as a mobile cannon turret with urban tires installed in his legs.
Pachirisu- She is a secondary Pokemon used by Kate that gives any Ranger the Sectro Static Armor which is a massive back pack that unleashes a powerful electro-magnetic E-shield that can reach a radius of a hundred feet max. As an Auxilery Zord she can replace Treecko as the left leg of the Palkia or Dialga Megazord and gives the Megazord the Electro Crash Stomp attack that sends a paralyzing seismic wave of electricity.
Absol- Aside from being Jack's henshin, she also gives Jack the Sectro Dark Gunblade which fires off pure dark energy in the form of energy daggers from its ionizer blade. As a Zord she is pretty much a robotic cat.
Heracross- He is a secondary Pokemon used by Jack that gives any Ranger the Sectro Javelin Driver, a small hover vehicle that has a powerful penitrating beetle horn that can pierce through armored Predacores at berserke speed. As an Auxilery Zord he can replace Absol as the right leg of the Palkia or Dialga Megazord and gives the Megazord the Lancelot Blade Kick attack which slashes a target with a retractable ionizer horn-blade attached to the leg.
Lugia- A secondary Pokemon that any of the rangers can summon to form the standard finisher module the Maelstrom Cannon, a poweful weapon that requires the combined effort of the five rangers to man and uses powerful pressurized air to deliver a devastating concussive impact that renders a Predacore vulnerable so that the Sectro Anti-Virus can be administered.
Entei- Aside from being Samus' henshin, he provides her arm cannon's Plasma Beam, a highly concentrated thermal heat energy laser with heavy attack strength. As a Zord he is a heavily armored feline robot.
Raikou- He provides Samus' arm cannon with the Wave Beam, an intense electro magnetic arc laser. As a Zord he is a feline robot with plenty of beam weaponry.
Suicune- He provides Samus' arm cannon with the Ice Beam, ironically the slowest beam but one with instant sub-zero cryogenic stopping power. As a Zord he is the fastest of the feline robots.
Lucario- Aside from being Link's henshin he gives Link the Sectro Master Sword and Sectro Hylian Shield, as well as the Sectro Fairy Bow which uses Aura to produce energy beam arrows. Unlike the other Pokemon's Zord forms, Lucario's Zord form is very much a Megazord in itself if only a lesser class, he can in fact seperate into five pieces that serve as substitutes the other zords needed for either the Palkia or Dialga Megazord, this allows the Palkia and Diaga Megazords to both be formed and fight side by side. His main weapon is the Lucario Sword and Lucario Shield. His finisher is the Aura Burst, a powerful gathering of aura energy that can take on any form possible (commonly a sphere or a wave) and be used to take down the targeted Predacore.
Groudon- The main function of this powerful Pokemon is to form the SectroBeastor Battle Train, a massive ground based heavily armored mobile transport that the Rangers use to drive into battle, it can run on a set railroad to its destination if needed but can also go off them and drive on any terrain if absolutely nessecary. It can actually carry the Pokemon Zords into battle fully prepared and charged up if needed.
Kyogre- The sole function of this powerful Pokemon is to form the SectroBeastor Battle Plane. It is very much like the Battle Train with the main difference being it is an aerial vehicle but can also go deep underwater withstanding unfathomable amounts of pressure.
Aside from their own Primary Pokemon's Zord forms, they also utilize the three Legendary Dragon Zords which actually form the base, battle armor and primary power core of the Megazords; Palkia Megazord; The first of the dragon Megazords, formed when the Palkia Zord combines with the Pikachu Zord (head), Piplup Zord (left arm), Chimchar Zord (right arm), Treecko Zord (left leg) and Absol Zord (right leg). Not the fastest of the Megazords but has the highest armor rating and has incredible attack strength, it's main weapons are the Dragon-Armor Claws and the Temporal-Space Sword. It's finisher is the Astral Plain Zentetsuken, teleporting the targeted Predacore into a limbo like dimension then using that dimension's energy to decimate the target.
Dialga Megazord; The second of the dragon Megazords, formed when the Dialga Zord combines with the Pikachu Zord (head), Piplup Zord (left arm), Chimchar Zord (right arm), Treecko Zord (left leg) and Absol Zord (right leg). It is the fastest of the two and relies on more tactical attacks and tricks to get the job done, it's main weapons are the Lucario Sword and the Chrono-Distortion Bow. It's finisher is the Time Arrow Rain, first it hits the targeted Predacore with a well placed chrono-distortion light arrow to freeze the target in time, then proceeds to go into close range using the bow's blade attachments to slice the target to shreds.
Giratina Megazord; The third of the dragon Megazords, piloted exclusively by the Entei Ranger on account that the zords needed besides the Ginatina Zord are the Entei Zord (torso), Raikou Zord (left arm) and Suicune Zord (right arm). It has the highest energy output of the three in terms of fire power, believe me this thing has some serious guns, among it's arsenal are the two main Spreader Beam Ray modules and the God Killer Buster Rifle. It's finisher is the Hellish Ray-Storm, a finisher that is rather self explanatory^-^. The other two Megazords can also transfer energy to the Giratina Megazord to boost its power for the second finisher Sectrobeast Blade, a massive energy beam formed into a sword weapon of impossible size that takes down any targeted Predacore in a single strike.
It is believed that the culprits behind this Predacore epidemic is in fact the infamous Team Rocket, of course the only evidence to suport this claim is in fact the two members Jesse and James, they've been spreading the disease with their own unique (and corrupted) Sectrobeast Binary Armor Tech as well as an army of brutish pig faced monsters called "Moblins". In actuallity, the two are not working on Team Rocket's behalf but are in fact under the command of a mysterious other worldly villain who calls himself "Ganondorf". Aside from the rogue Team Rocket operatives he has several lieutenents under his command including the sentient master computer organism simply known as "Mother Brain", the vile twilight wizard Zant, the infamous and brutish Italian thief "Wario", a dreamscape knight known only as "Meta Knight" and a fiersome dragonoid from outer space code named "Ridley".
The overal theme I was going for in designing these rangers is both animals and technology, like putting Gekiranger and Boukenger into a blender. I tried to make them look like the Pokemon they represent but also give them a distinct almost field military unit-like design. As always esults may vary.
And yes, I am aware that Lucario is not purple, at least not normally. Since there's apparently a Red Garadoss), who's to say that Link's own Lucario is not colored dark Purple instead of blue? Besides I like the idea of a purple Power Ranger.