Killer Clown

The Killer Clown was a minor villain that appeared in the Buffy episode "Nightmares" - as his name suggests he was a clown conjured by the fears of Xander Harris and although not appearing for very long nor being much of a large-scale threat was nevertheless an antagonist worth noting.

History
The Killer Clown was formed from the fears of Xander Harris, who as a child had been chased by a clown during a birthday party and developed a phobia as a result - when the town's nightmares started coming to life Xander's fear manifested itself as a manic clown with a knife that proceeded to pursue Xander with the presumed intent of killing him.

At first Xander was too scared to confront his childhood fear and fled but eventually had enough of the clown and punched him in the face - when Xander did this the clown disappeared and was never heard of again, a symbolism that Xander had finally gotten over his phobia (or at the very least learnt to cope with it).