How will a horse react to anabolic steriods?

Answer 1

I'm not sure of the answer to your question, but vets do use steroids to treat horses. My horse was on prednisolone and prednisone for about 5 months to control the purpura after strangles.

http://www.hematologic.niddk.nih.gov/pubs/itp/itp.htm

Answer 2

I don't know, but if you give them to him/her you could kill him!! It is also against the law to give animals steroids. And it is worse if you race your horse. You and your farm would be banned from the track for life. And would recieve a fine, and probably your horses would get taken away.
Don't do it a horse is perfect the way it is.