your horse should be fine if he didn't have a hole lot but be careful with the feeds for if your horse gets medicated chick feed it could die as well as any other kind of animal like dogs, cats, or even adult birds for the medication to them is toxic.
My pony will some times escape from her pen so i keep the feed shed locked up
You know, I used to think horses pooped a lot. Until I got chickens.
Edited to add:
He did get medicated chick food. I called the vet, and he said he couldn't find any info on amprolium and horses. He said to call the feed company. The feed company said I'd have to feed Jay 5#/day for a month before I saw any effects--and even then, the medication wasn't "known" to be toxic to horses. He got a total of one pound.