Been lurking on AO3 for a while on this exact topic. The fics that nail it for me are ones where the animal traits aren't just cosmetic—they fundamentally change how a fight works. Like, I read this one where Bakugou was a honey badger, and the action scenes were insane. His 'explosions' were reimagined as this foul musk spray, and his fighting style became this relentless, digging, tearing chaos that felt totally true to both the character and the animal. The instinct to never back down, to go for the tendons, it made the combat visceral in a way human-versus-human fights in BNHA sometimes aren't.
Another standout was a pod of orca!Class 1-A fic, set in an oceanic AU. The pack hunting strategies during the 'USJ' equivalent were brilliantly plotted. You had Midoriya as a clumsy young orca figuring out how to tail-slap, Todoroki using temperature blasts to create currents, and the villains as aggressive sharks or giant squid. The action was all about three-dimensional movement, echolocation, and using the environment, which made every encounter feel fresh and instinct-driven, not just quirks with fur.