I bought a wormy apple to give to my horse as a treat to see what It does but I can't give it to them. The treat option is nowhere to be seen. (This is outside explore on the horse page).

Pretty sure the wormy apple is to decrease a horses morale in explore. Not to treat for horses in general

Nope. It can be a treat. Go back 1 news article. It said they can now be used as treats but your horses may not appreciate it:)