My mare got into a whole bunch of rabbit food last night at my barn.should I be worried?

Make sure that you keep a close eye on him, on his teeth, ears, tail, and feet. If his ears start to get longer, his teeth become more rodent like, his tail get smaller and fluffier, and his back feet become longer, he just might be turning into a rabbit.

On a more serious note, make sure he's drinking plenty of water and he should be fine.