Devouring Maggot

Devouring Maggots are critters found in the catacombs underneath Wyrmskull Village. Go in the cave above the pit with the Bloodthirsty Worgs. Down the stairs, take a right at the Valkyries, down more stairs, all the way to the bottom.

Among these can spawn Devouring Maggot companions, that can only be captured through pet battling. These can also spawn as secondary pet battles.

Pet Journal
''Howling Fjord teems with the ever shifting cycle of life. When a creature dies, its body is quickly consumed by maggots, which in turn pass those nutrients on to a number of other creatures.''

Achievements
Killing one of these is required for Pest Control.