A Gremlin is a hostile mob added by the Better Dungeons mod. They generate in any dungeon type with Default Mob defined as the mob type and the structure is between 2000 and 3000 blocks from spawn. They wear any type of armor and can be a Warrior, Healer, Archer, or Grenadier. They usually appear in groups. You'll be slain without good protection as the Gremlin warriors will chase you and the archer and grenadier will attack from a distance.
Gremlins prioritize riding mountable animals before attacking. A Gremlin's movement speed will increase once mounted. Sometimes attempting to escape any dungeons on ground, they'll give chase on mounts.