I've only seen a handful of them, and it was only in the colisseum. And it was funny because once it defeated a really hard monster in there, one that you're not supposed to beat I guess, and got a rubbish item. They are pretty useless though. I dont know the exact probability of them coming out, but they seem pretty rare. No one was gonna want to choose Fight while that low in life. They'd be healing or something. But I think I saw some game genie codes so they always come out, just for fun.