This was requested by a fan and I can't believe I didn't think of it before. The monsters of Boukenger were inspired by past robos, Jetman, Changeman, Goggle Five, Dynaman, Bioman, JAKQ and Turboranger got no love. These are in order of the robos/megazords through the years, from 2005 down. Carranger, Megaranger, Flashman, GoGoV and Dekaranger only got 1 monster/robo to get something based on them when they had more than 1 robo.
The Karths/Chillers seem to be the only villains who are not based on anything. I could have a million theories on what they resemble but I haven't found any official document saying what they resemble. I have a magazine with the concept designs and there is no mention. And of course, Flurious isn't based on anything either, because he was made exclusively for Power Rangers.
Modified Mirage Beast Gordorum (Vulturus) / MagiKing (Titan Megazord)
The monster has a unique look, I like the shapes. The wings look similar and it has the M on the chest. It looks similar structure wise. The horns in the front and the shape of the head emulates the hat.
Mirage Beast (Vulturus) / MagiLegend (Manticore)
The wings on the monster is much bigger, the center thing doesn't look anything like a lion. It has a crown on its head.
Quester Robo Turbo (Rapidly) (Jet Robo) / DekaWingRobo (SWAT Megazord)
The only monster based on a robot from Dekaranger/SPD.
Dembey / Abarenoh (Thundersaurus)
Dembey's horns, shoulder pads and feet look like the robo, but everything else I don't see the difference. Dembey is actually based on a TV show mascot (not a real one) so it had to simplify.
Prince (unnamed monster) / KillerOh (Stegozord)
The leg flaps looks similar, the shape of the monster is similar. The wings on the face are similar, even the shape of the gold thing on his chest. The color scheme might not be in the same spots as the color scheme of the robot, but it is similar.
Zorad (Lavadactyls) / Top Galer (Dragozord)
It is clearly inspired and based on the robo, I think they even used the same movements.
Dagargin (Magmador) / MaxRyuuOh (Mezodon Megazord)
The colors are definitely inspired but it doesn't really look like it.
Madness Weather (Weather Machine Monster) / Bakureno (Replicant Megazord)
The shouldar pad, shield, arms, legs and the head are similar. The middle is not the dinosaur but it has a design somewhat similar.
Takumigami (Bombardo) / Gouraijin (Thunder Megazord)
It clearly has the center stag horns thing. It is more inspired by, because it has a similar head but different design and has leg cuffs that look a bit like the robos' legs. The shoulder pads are nothing alike.
Ouga (Crazar) / Gao Hunter (Predazord)
The monster is based on the evil mode, hence the horn on the head. it clearly looks like a wolf. It has diamonds on the knees which seams to be inspired by the kneecaps of the robo. The weapon looks nothing like the weapon of the robo.
Hyde Gene / GaoIkarus (Isis Megazord)
From the movie, this monster has the wings and horns for sure. And the 'Eyes' turned into red gems.
Hyde Gene / GaoKentarous (Pegasus Megazord)
It has the wings, the bottom, where has a mouth, it looks like the lion but the tail... i guess it is the tail from the lion.
Magi (Tyzonn) / Time Robo Alpha (Time Force Mode Red)
The face looks similar and the face, but everything else doesn't look anything like the robo, it looks like it was inspired slightly. The feet, how one part comes out, the monster has those side tentacle things.
Kanadegami (Amplifier Monster) / V-Rex Robo (Quantasaurus)
Time Demon God Chronos / Providus (Transwarp Megazord)
The monster has one amplifier that looks like the shoulder pads of the robo. Other things that are similar are the feet, the right arm missiles, and three horns on the head.
Dragon Grand (Cybernetic Rex) / V-Rex (Qunatasaurus)
Clearly the Tyrannosaurus is present in the monster design and the arm cannons too. But the top part of the monster is new. But everything else looks exactly like the robo.
From the Boukenger vs. Super Sentai, I don't see it.
Akutagami (Garbage Monster) / Linerboy (Max Solarzord)
Because of the nature of the monster, it looks like garbage, so doesn't look the same as the robo, maybe mildly similar.
Dryken (Volcan) / Gingaoh (Galaxy Megazord)
The head horns, the shoulder pads, V center, ape face and lower legs are similar.
Girad (Dragonizer) / GigaPhoenix (Stratoforce Megazord)
Zard (Moltor's Robot) / Giga Rhinos (Centaurus)
It clearly has the shouldar pads in common but the rest is more inspired, because it doesn't look exactly like it.
The monster has a more bloated look but it is similar. It has the face, waist, and legs.
Naga (Bullox) / BullTaurus (Torozord)
The horns, feet, shoulder pads and chains on chest and weapon are clearly from the robo. They made the gem in the waist into a bigger one on the chest.
Zukangami (unnamed) / Astro Galaxy (Astro Megazord)
Hmm.. The tie kinda looks like the M. The arms are similar, the circle thing on the monster's left arm looks like the shield of the robo. He uses the same attack but I think he is more 'inspiried' by the robo than anything.
Kawazugami (Big Mouth Monster) / VicTrailer (Artillatron)
The monster is inspired by the robo and I like the final design. I am not crazy about the robo though. The only thing I see that is inspired by is the center mouth thing. And the legs look like the four squares but stylized. The only robo from Carranger/Turbo.
Yaiba (Kamdor) / Invinicible Shogun (Shogun Megazord)
He is clearly inspired by. I like his designs. I like both designs actually. The only similarities I can see is the shouldar pads and maybe Yaiba/Kamdor's helmet.
Super Shizuka of the Wind (Kunoichi Monster) / Daishogun (Ninja Mega Falconzord)
She did indeed looked like the robot, the wings from the 'falconzord', the eyes on the crotch to look like the frog, and the headdress. The arms aren't the same though. You can't see the boots but they don't look like the robot's feet. Shizuka was based on the Ninja Megazord and this super form was based on it with the falcon.
Demon Bird (Miratrix) / Tsubasamaru (Falconzord)
Both are giant birds. The claws and talons are similar.
Gai (Mig) / Ryuusei'Oh (Red Dragon Thunderzord in Battle Mode)
I see he has the lines on his legs, the V shape on his waist and the shoulder pads but the rest seems to be left to the imagination.
Quester Gai (Mig) / Dairen'Oh (Thunder Megazord)
He clearly has the green gem in the center and the shoulder pads but the waist doesn't look like it and the helmet is much more elaborate.
Rei (Benglo) / Won Tiger (White Tigerzord)
The tiger face on the chest is the most obvious part.
Quester Rei (Benglo) / Kiba-Dai'Oh (Mega Tigerzord)
The face is no where near the look, maybe the gem but that's it, he clearly has the shoulder pads.
Hyouga (Cheetar) / DaiMugen (Tor)
He clearly has hexagons on his chest like the robo and has the horn and gem on the battle mode head. His wrists has covers that looks like the paws of the turtle. The leg cuffs look a bit like the feet of the battle mode.
Talong (Blothgar) / Daijinryuu (Serpentera)
He is clearly inspired by Serpentera, they look so much alike (shoulderpads, shape, feet, etc.), except the monster has a more animated look and the hip pads. The ying-yang simple does look like the gem on the robo's chest.
Ryuuwon (Moltor) / Daizyujin (Megazord)
He is clearly inspired on the original Megazord as he has the horns, shoulder pads and teeth on his chest.
Jaryuu (Lava Lizards) / Dragon Ceasar (Dragonzord)
I don't see it.
Lindom (Scaletex) / Gouraijin (Dragonzord in Battle Mode)
Dold (Giant Sea Monster) / Ultimate Daizyujin (Ultrazord)
It has the shoulder pads, the chest seems to have a similar design and the hips has that pad thing. I would not directly think of the Dragonzord when seeing it but yeah at further expectation, it is clear.
It looks like it was more inspired by the King Barchion/Titanus part but not the feet. The circle on the waist is like Dragon Ceasar/Dragonzord's shield. The dragon face and neck looks like Titanus and so was his side arms.
Quester Robo Elite (Commander) (Commando Robot) / Five Robo
Shirubegami (Sextant Monster) / MaxMagma
It is based on Fiveman's Super Five Robo and Five Robo, clearly by the helmet, chest and shoulder pads. Jetman robos got no love.
I don't think it looks anything like Max Magma of Fiveman.
Kyoshin Gagadomu (Flurious' Robot) / Super Live Robo
It is much more bloated and doesn't remind me much of the robo. I hate the design of the Boukenger evil robo.
Homunculus with Quester Jet Over (Agrios)/Great Five
The helmet, chest stuff and shoulder pads clearly look like Great Five from Maskman.
Quester Robo Blaze (Erupt) (Centurion Robot) / Sun Vulcan Robo
It clearly has some V-parts and the legs are similar but the shoulder pads are moved to next to the head.
Quester Robo Radial (Advance) / DaiDenzin
Gaja / Varriccune
The helmet clearly looks modeled after it, I like the design of this Quester Robo more than the original robo from Denziman.
WTF? This is the strangest inspiration, I do not see it.