Yeah I just had one of these born off a stalk, I think it's a premature ko. They can't be scolded out of shithead, and if you pick them up all they can say is "yu."
Finding one is random, as with any other type. I think the big-eyed weird ones were a bug, though, so I'm not sure they're still around.
Currently you can abyuse a normal yukkuri (with the help of OJ so they don't die) into yukkuritis, which basically causes them to act like a premature yukkuri.