I love vanity plates! I have a horse vanity plate but in VA you can have up to 6 letters. 5 letters is a little hard. How about:
TBGLD (or alternatively, WB for warmblood, TW for Tenn. Walker)
WOAH (alternative to whoa)
If your horse has a short name a lot of people make the name their plate, for example I know of "SONNY" and "SONEE" and "BENTLY" in my area. Hope this helps!
Maybe not appropriate for a personal vehicle, but a friend of mine bought a horse trailer years ago and the previous owner just left the old tag on it. It read: 2NAGS (2 horse trailer, natch) I absolutely loved that and swear I'm going to use it one of these days...
They came out with equine tags here in Georgia several years ago, but you can't specialize them. Can only get whatever next number sequence is.