Bulgallian sludge rats (or simply Bulgallian rats) were an alien animal.
In 2364, Wesley Crusher, who was undergoing the Starfleet Academy entrance exam, considered bringing up the images of Bulgallian rats in the holodeck in order to prepare for the upcoming psych test. Later, after Rondon, a Zaldan, collided with Wesley in a corridor, he referred to Wesley as a "Bulgallian sludge rat." (TNG: "Coming of Age")