I've seen those before. Whatever you do, don't let a vet cut them out! That just makes them go into hyperdrive. :)
I've never had trouble getting rid of that type of wart. What we do is get some Crest toothpaste (the regular kind, not the gel) and dab a generous amount on the wart. Re-apply as needed. Be sure to check everyday because other horses like to lick it off sometimes. The wart will eventually just drop off. If it has a really deep root it might take a month, but that darn thing will crumble up, sometimes breaking off in pieces, and then be GONE. Once gone, I've never had them come back using this simple remedy. Give it a try. It can't hurt.