When do they appear: Once you get to Devil's Pit, you will start facing these monsters.
This enemy is very tall and can crawl around on the ceilings. They have long reach and can surprise you by dropping down. Sometimes you will be faced with multiple. These enemies are one of the most rare in the game, so fight then every chance you get if you are going for this achievement. This is the hardest enemy based achievement in the game.
Glitchy: It seems to only count if you kill or incapacitate these enemies while they are on the ground (not shooting them while they are on the ceiling). Some also say they randomly spawn on the overpass after St. Maria's. Some have seen a bunch of them spawn, while others have seen 0. There are 2 side quests that you can find them in, Dead Man's Hand and in the Monroe Cemetery when you finish the Art Collector quest. You can avoid all of these locations if you just take care of the ones you see.