Every animal that dies during toxic fallout will be instantly rotten (at least those, who are affected by poisoning. Animals that belong to manhunter packs won't rot instantly if they are not affected by poisoning). I don't know if this goes by design for animals that die because of the fallout (perhaps Tynan decided that the meat yield during fallouts is to high), but I don't think animals that get killed by colonists should rot instantly.
Currently indented by design so far i know.
But i agree, the rotting should be changed depend on the toxic state of the copse.
When the corpse just got a minor/light toxic he should stay longer fresh then an animal that die from the toxic fallout.
But that's part of the suggestion forum.
It is by design. Thanks for reporting.