Chimerical Trait Acquisition is a rare condition, thankfully. What happens is that somehow, when the mutation kicks in and the DNA is in flux, foreign species' DNA gets intermingled with the expressing mutant's DNA, and the resulting hybrid resembles a mix of the two species. The problem is that the foreign DNA becomes dominant, and seeks to overwrite the original DNA, and unless the mutant has a shifter trait to allow the body to survive this is almost invariably fatal. And in those cases where not fatal, the mind is still wiped when the brain's DNA is rewritten, so it might as well be fatal, because no one is home.