A normal-size balloon of helium can lift about 10 grams. Hares can be about 4 kilograms. Totally doable but that's a lot of time, balloons and helium to just waste!
Anyway, here are some more species and approximate adult weight when alive:
'sure that happened' but moving on from that and taking it from a comedic stance: The absolute chaos of balloon harnessed roadkill just floating into traffic idk, fucking amazing mental picture there, lmfao. Or just dropping in front of a school bus and splattering on the windshield, children screaming, etc. There's def a couple comedy sketches that could be done there.
So, Sir, you're trying to tell me the deer that's lodged into your grill was already dead and "floated" into the road. Sure. Let's go down to the station for an extended drug screening.
I am so fucking tempted right now. To the point of maybe next time I'm at a party supply store. To purchase one of them helium to go tanks and couple packs of balloons. I know I stand a slight chance of absolutely fucking up some ones day. But before that dread will kick in.... I WILL be absolutely laughing my fucking head off with the guys to the point of needing to try to stop the funny because I'm gasping for air.
You're going to want to run a LOT of math before you finish ordering the gas lol. I'm thinking a bird would be optimal for the hollow bones, but it also wouldn't be confusing, just ominous. Mouse?