Normal mirelurks are crabs. Mirelurk kings are snapping turtles.
That makes sense. :rolleyes:
Anyway, here we go cats, the big list of Fallout critters and how they came to be:
Giant Ant - regular ant, mutation from nuclear fallout.
Fire Ant - giant ant, with fire mutation.
Bloatfly - blow-fly, mutated.
Brahmin - cow (Brahman breed possibility), mutated.
Centaur - result from The Master's experimentation with FEV. Humans + dogs + various other creatures dipped in FEV vat.
Coyote - coyote, very little mutation.
Dog - dog, very little mutation.
Wild dog/vicious dog - dog, little mutation, sickened by radiation and elements.
Cyberdog - dog with robotic enhancments.
Deathclaw - genetically engineered creature by US government. Jackson's Chameleons, genetically modified with DNA from other creatures. Originally created before the Great War by the United States Military in order to replace humans during high-risk close-combat search-and-destroy missions. Post-war, The Master also experimented with existing Deathclaws, which made them more powerful and what they are today.
Floater - most likely FEV-infected flatworm; further enhanced as a result of The Master's experimentation with FEV.
(Little) Gecko - gecko, mutated.
Golden Gecko - gecko, mutated. It is assumed they are a result of geckos being exposed to toxic/radioactive waste.
Fire Gecko - gecko, mutated. Can breathe fire due to unknown toxins in their gut.
Ghoul - human, mutation. Limited exposure to radiation for a longer period of time sometimes causes humans with an unknown genetic characteristic to transform into ghouls, although rare cases of faster transformations into ghouls are also not unheard of. Exposure to radiation typically results in sickness followed by death for the average human being, however, the genetic x-factor that will lead to mutation into a ghoul upon exposure instead of the typical deadly outcome remains unknown.
Glowing One - ghoul, has so much radiation that they glow.
Human - human, little to no mutation. All wastelanders has some degree of mutation, where as vault dwellers, depending on what vault they were in, have the chance of no mutation or radiation exposure whatsoever. Vault dwellers of Vault 13 have no mutations or exposure to radiation.
Iguana - iguana, survived the great war, but are common food for wastelanders.
Mantis - mantis, mutated.
Mirelurk - horseshoe crab, mutated.
Mirelurk King - snapping turtles, mutated.
Mole Rat - rodent, mutated. West coast has a variation covered in fur, east coast are mostly bald. The mole rats in the Capital Wasteland (and possibly the other mutated rats in Fallout and Fallout 2) are the result of an American government program to develop an invasive organism that could be planted in China, where its aggression and fast breeding would help undermine the Chinese war effort.
Nukalurk - mirelurk, mutation from exposure/consumption of large amounts of Nuka-Cola Quantum.
Pig rat - rodent, mutated, reminicent of pigs and rats, hence the name. Possibly west coast only.
Radroach - roach, mutated.
Radscorpion - Emperor scorpion, mutated. Their poison grew more potent when they mutated rather than becoming diluted. Small and giant varieties.
Giant rat - rat, mutated. Simply just a bigger rat. Have little mutation; see mole rat, pig rat for one's with more significant mutation effects.
Spore plant - venus-fly-trap plant, mutated.
Super mutant - human, product of FEV experimentation/infection/exposure. Two different sources of super mutants: Mariposa Military Base on the west coast, and Vault 87 on the east coast. The two populations have separate histories and origins, and there are no known accounts of any interaction between them.
Super Mutant Behemoth - super mutant variation. Seems to be only found in the Capitol Wasteland.
Wanamingo - animals (no certain origins), mutated. Often called "aliens" because of their appearance. They were designed as FEV-tailored weapons for waging war on other countries and they escaped into the wastes.
Wolf - wolf, very little (possibly no) mutation. Seen on west coast, possibly survivors from Yellowstone reintroduced packs.
Yao Guai - bear (possibly mainly black bear), mutated. Name simply means "monster" in Chinese.
This is pretty much it, I think. Source: http://fallout.wikia.com/wiki/Fallout_Wiki