no really, they just go super fast, few of us have seen them, they'll just chase you down until you either go down a cliff (where they'll hit you a few times on the way down/up) or get KO'd
Can confirm, have encountered these multiple times. One fast ant among the normal ones, have been KO'd by not noticing before its too late, then the rest of the hill come and get one hit in each.
From all the hills I've encountered, I'd say I've seen it maybe 1/25 times.