Somebody, I'm pretty sure it's Loki, hits the Avengers with a spell that draws out animal characteristics for each of them. From what I remember, Natasha is a snake(?) of some sort, I think Bruce was a rhino, either Thor or Steve was a dog. They got some physical characterisics, like Bruce had denser gray skin, Natasha had scales, and also behavioural characteristics were magnified. Clint got feathers in his hair and like enhanced bird-senses or something. It all started to fade after a few days, except for Clint's, which didn't fade at all, and they figured out it was because Clint was comfortable with the changes and accepted them. and then I'm pretty sure he used the enhancements to kick Loki's ass or something. Also I think it was Clint/Coulson? But I could be wrong.
Help me find the pretty story please? :)
FOUND: Let Them Call It Mischief by infiniteeight on AO3.