Throughout Madagascar, they are shown to terrorize the lemurs and attempt to eat them. According to Julien, "they're always annoying us by trespassing, interrupting our parties, and ripping our limbs off." When they do speak, they put "fossa" in front of every word: ex. "Fossa hungry, Fossa eat." They are terrified of Alex due to him being a lion and being much more physically powerful and intimidating. In the end, thanks to a setup performed by his friends and the penguins, Alex scares them away making sure that the fossa never bothered the lemurs ever again.
In The Penguins of Madagascar, episode "The Terror of Madagascar," a baby fossa appears. This little guy is innocent, but Julien thinks he is trying to eat him. The fossa is revealed to think Julien as his father and saved him from Savio.
The Fossa appears as the secondary antagonist in All Hail King Julien. King Julien's uncle, King Julien XII had previously kept the kingdom quiet in order to keep the fossa away.