Make your scary stuff and then work backwards.
Dutch is right. For example, some people might find skinny hippos running all over the place to be scary because hippos are supposed to be fat and in the water. Is an aquatic giraffe scary? How about a 2000 lb cheetah that is not fast? No? OK: predatory oxpeckers that attack in a gang! Is that a little bit spooky?
Start with your scary then justify it with mutations. Or bioweapons? GMO freaks bred as status pets for Nigerian oil oligarchs? Or they already had the triffids in oil farms and then they got loose? Because the fact of the matter is that 100 years is not enough time for random evolution to radically change a large vertebrate species, radiation boost to mutations notwithstanding.
Don't get bummed though! Low scifi does lots of things that don't hold up to hard scrutiny and I am thinking of many, many scifi scenarios from The Time Machine to Long Afternoon of Earth to Fallout, all of which featuring creatures which have mutated to sinister and freaky and awesome new forms. Might I suggest (for variety) also the hottie new forms, Weena of the Eloi being a fine example of easy-on-the-eye mutantry?
The intention is for the reader or player to marvel at the freaky and awesome, not get all egghead about radiation mechanisms and evolution time.
Your trick is not figuring out realistic genetics (ho hum) but figuring out scary and sinister in a new and engaging way.