Spoiler They are results of genetic engineering before the bombs dropped, so they are not actually a product of radioactive mutation. They were designed to replace soldiers in some high risk situations
^This on deathclaws
scorpions were popular pets at the time, and were abundant in pet stores which explains why they are everywhere in D.C.
Mole rats- From a terminal in the Citadel, were a government experiment to be bred and shipped over to china to undermine their cities. They were gentically made to burrow into almost anything.
Bloatflies- Mutated Horse flies
MIrelurks- Blue horseshoe crabs
Mirelurk kings- Snapper turtles
Giant ants- Mutated from your average garden variety ant.
Centaurs- Mutated FEV subjects originally created by mixing humans and other animals in the FEV at once.
Yao Guai- Mutated brown bear (or was it black bears?)