So I ended up killing most of them. I have 5 in a pen, and one still floating. That last one survived FIVE hits from my power 5 flame bow, so he must be one tough horse. One of these days I'll go collect him (he's pretty far out so it will take a long time to push him to shore).
Err...I've yet to encounter one so I could be wrong but, I thought trap horses were spanwed tame and could be saddled/ridden/led just like any other horse. Is that not right?
[EDIT: I guess they can't swim while mounted, though, so riding them off the island wouldn't do much good. If you can't lead them like a regular horse, then I don't know...]