Lots of options from demons, but none that fit to your satisfaction. Felguard have the size, but a distinctive single horned helm. Lesser Man’ari Eredar - Wrathguard as they are usually grouped - have the right physiognomy except a tail too large to escape notice beneath a robes. And both Nathzerim Dreadlords and greater Man’ari Eredar could for the bill, but they’re just not the kind of demons to be sneaking around in a swamp, jumping on groups of cadets. If you’re surrounded by demons of that calibre, the third invasion of Azeroth is underway - and that seems unlikely.

All in all, it strikes you as unlikely the cloaked figures are demonic in nature. But between the robes and the track-covering magic, they have a reason to disguise their presence.