People who school/punish their horse AFTER the class if it did bad. Sorry, if you're going to show you should prepare and train BEFORE the show!
PETA extremists. Just last weekend I was walking through a cattle barn at the fair and there was this crazy lady making her kid walk around in some cow poo and taking pictures of it. Okay, maybe it's more of a sanitation thing than a PETA thing, but I hate those extremists too!
People who waste/ruin horses. I've seen people go out and buy AMAZING horses and they just sit in the pasture.. or people that buy amazing, talented (and forgiving!) horses that yank on their mouths or ride in crazy spurs and stab the horse every stride. :(