Diictodon are small burrowing mammal-like reptiles.
A group of over fifteen individuals appear to be traversing air vents in a hospital, chewing through the electricity that the team will need to remove them. Abby catches one which sends of a distress signal calling them all back through the anomaly but one drops from the roof (Sid) and lands inside a trolley Connor is pushing. A single Diictodon tried to escape through an anomaly, which disappears. It later becomes apparent that Abby and Conner adopt two to keep Rex company. In episode 3.5 they are called Sid and Nancy and are living with Connor while Abby's brother is staying. When Connor was staying with Lester, the two caused him much grief.