Yeah, Yawn sounds plausible indeed, but so far all the pulses we've seen are strongly related to what they produce (tangrowth = growing vines everywhere , muk polluting everything because its the epitome of sludge, mr mime putting up a barrier because its a mime... camerupt trying to erupt a volcano.. avalugg being an iceberg thing itself creating a giant ice-block, abra famous for teleporting and so on) , so I think whatever the pokemon is, will be strongly related to, or famous for putting things to sleep