A mugger is a human monster in SLASH'EM and UnNetHack. They are not randomly generated and only appear on certain special levels, often around shops. In SLASH'EM, four of them appear in the shopping mall (if it exists), and one is guaranteed to appear in Grund's Stronghold. In UnNetHack, muggers appear in the Town.
The mugger's name implies that it has the ability to steal items, like a monkey. This theft attack is implemented in UnNetHack, but not in SLASH'EM. Otherwise muggers are substantially the same in each variant.
Muggers in SLASH'EM are not much of a threat, except inasmuch as one might find and use an attack wand or a highly enchanted weapon from a shop. This is relatively rare, however, and most of the time they are somewhat nonthreatening.
Given their habit of collecting items, they can be useful in stealing a few items from shops for you, which at the very least can then be sold back to the shopkeeper. For this reason, it is well worth it to make sure not to kill them in the shops in the mall, but rather wait for them to come outside, so the shopkeeper will no longer own whatever items they are carrying.
UnNetHack muggers are a little more dangerous because of their ability to steal from the player's pack, but otherwise they are no stronger or harder to kill than their SLASH'EM counterparts. A player in this variant may prefer to kill any muggers that appear, from a distance if necessary, before they can reach his or her pack, rather than spare them in hopes of reaping the benefits of their shoplifting.
Encyclopedia entry (UnNetHack)
"A man says to another man, 'Can you tell me how to get to Central Park?' The guy says no. 'All right,' says the first, 'I'll mug you here.'"