Unused creatures.

[So_So_Man]

or the male could evolve into a useless (stats-wise) pokemon with a very limited breeding pool but a huge move pool

[Java]

I don't know about you, but I've always known Typhlosion as the Volcano Hamster.

Uhh...

[The Riddler]

Uhh...

Quilava looks like some kind of gerbil.

DOING GEN2 NOW.

[The Riddler]

Adding in bugs that aren't bug type. This list is by no means official.
Edit: Also did ones from later generations that I had on my mind while I was at it.
[spoiler=All creatures up to Celebi...unless I missed something...]
Bee/Wasp - Beedrill, Combee, Vespiquen
Bagworm - Forretress, Burmy, Wormadam
Butterfly - Butterfree, Beautifly
Beetle - Pinsir, Heracross
Caterpillars/Worms - Caterpie, Weedle, Wurmple
Cicada - Nincada, Ninjask, Shedinja
Cocoons - Metapod, Kakuna, Cascoon, Silcoon, Pupitar
Cricket - Kricketot, Kricketune
Dragonfly - Yanma, Yanmega
Firefly -  Volbeat, Illumise
Gnat - Venonat
Ladybug - Ledyba, Ledian
Mantis - Scyther, Scizor
Moth - Venomoth, Dustox, Masquerian, Mothim
Pond Skater - Surskit
Scorpion - Skorupi
Slug - Slugma
Snail - Magcargo
Spider - Spinarak, Ariados

Reptiles/Amphibians:
Alligator - Feraligatr
Axolotl - Wooper
Chameleon - Charmeleon
Cobra - Kobra
Crocodile - Totodile, Croconaw
Frog - Poliwhirl, Poliwrath
Giant Salamander - Quagsire
Salamander - Charmander
Rattlesnake - Ekans
Tadpole - Poliwag
Toad - Politoed
Turtle/Tortoise - Squirtle, Wartortle, Blastoise
General Lizard - Lickitung


Birds:
Birds of Prey - Pidgeot (resembles Falcon or Eagle), Fearow (resembles Vulture)
Crow - Murkrow, Honchkrow
Dodo - Doduo, Dodrio (though they resemble Ostriches more)
Duck - Psyduck, Golduck
Owl - Hoothoot, Noctowl
Penguin - Delibird (also, Santa)
Pidgeon - Pidgey, Pidgeotto
Quail (?) - Natu
Sparrow - Spearow
Wild Duck - Farfetch'd


Rodents/Small Animals:
Badger - Typhlosion
Bat - Zubat, Golbat, Crobat
Echidna - Cyndaquil
Ferret - Furret
Mole - Diglett, Dugtrio
Mouse - Pichu, Pikachu, Raichu, Azurill, Marill
Rabbit - Nidoran M/F/o/a, Nidoqueen/king, Azumarill
Raccoon - Sentret, Zigzagoon
Rat - Rattata, Raticate
Shrew - Sandshrew, Sandslash
Weasel - Quilava, Sneasel, Weavile

Feline/Canine:
Artic Wolf - Suicune
Beagle - Smeargle
Bulldog - Snubbull, Granbull
Cat- Meowth, Mew
Doberman - Houndour, Houndoom
Fox - Vulpix, Ninetails, Abra, Kadabra, Alakazam
Large Dog - Arcanine, Entei
Persian Cat - Persian (though it resembles a Siamese more)
Puppy - Growlithe
Tiger - Raikou
Mix of Feline/Canine: Eevee and all Eeveelutions

Sealife:
Anglerfish - Chinchou
Carp - Magikarp
Clam - Shelder
Coral - Corsola
Crab - Krabby, Kingler
Goldfish - Goldeen, Seaking
Jellyfish - Tentacool, Tentacruel
Octopuss - Octillery
Oyster - Cloyster
Pufferfish - Qwilfish
Manta Ray - Mantyke, Mantine
Remora - Remoraid
Seahorse - Horsea, Seadra, Kingdra
Seal - Seel, Dewgong
Starfish - Staryu, Starmie

Large Wildlife:
Baboon - Mankey, Primeape
Boar - Piloswine
Bull - Tauros
Cow - Miltank
Deer - Stantler
Elephant - Phanpy, Donphan
Giraffe - Girafarig
Grizzly - Ursaring
Horse - Ponyta
Kangaroo - Kangaskhan
Monkey - Aipom
Pig - Swinub
Rhynocerus - Rhyhorn, Rhydon
Sheep - Mareep, Flaafy
Teddy Bear - Tediursa

Mythology:
Baku - Drowzee, Hypno
Banshee - Misdreavus
Dragon - Charizard, Dratini, Dragonair, Dragonite
Fairy - Cleffa, Clefairy, Clefable, Igglybuff, Jigglypuff, Wigglytuff, Togepi, Togetic, Togekiss, Celebi
Ghost - Gastly, Haunter, Gengar
Leviathan - Gyarados
Phoenix - Ho-Oh
Rukh - Articuno
Ryūjin - Lugia
Suzaku - Moltres
Thunderbird - Zapdos
Tsuchinoko - Dunsparce
Unicorn - Rapidash
Witch - Mismagius

Prehistoric:
Dinosaur - Bulbasaur, Ivysaur, Venusaur, Larvitar
Ammonite - Omanyte, Omastar
Brontosaurus - Chikorita, Bayleaf, Meganium
Eurypterid - Kabutops
Mammoth - Mamoswine
Pterydactyl - Aerodactyl
Trilobyte - Kabuto
Tyranosaurus - Tyranitar

Other:
Human - Machop, Machoke, Machamp, Hitmonlee, Hitmonchan, Mr. Mime, Jynx, Tyrogue, Hitmontop, Smoochum
Heiroglyph - Unown
Pokeball - Voltorb, Electrode
Punching Bag - Wobuffet
Rock/Boulder - Geodude, Graveller, Golem
Totem Pole - Xatu
Waste - Grimer, Muk

Human-Created:
Mewtwo
Porygon, Porygon2, PorygonZ

No intercourse ing idea:
Slowpoke, Slowbro, Slowking
Cubone,  Marowak
Koffing, Weezing
Chansey, Blissey
Electabuzz, Elekid
Magmar, Magby
Ampharos
Snorlax
Wynaut
Skarmory[/spoiler]

[JrDude]

There is no such thing as a "king ant"
Though I was thinking an evo for a male ant could be a like a forest ant or whatever they're called, you know the ones that can fly? (A Bugs Life), but then when I remembered that movie, I was like "durp, only the female ants can fly" so reading this was likely a waste of your time, even though your time is likely wastable, wait, are you still reading this? stop reading these sentences... SERIOUSLY NOTHING IMPORTANT IS HERE, wow, I really am wasting my time here by writing this, but so are you for reading it, are you really that bored? 'cause I am.