Fossa had a minor role in Episode 2 - Jungle Area.
Role in the Plot
Fossa is shown at the beginning of the showcase of the variety of Friends in the Jungle Area. She converses with Kaban and Serval for longer than all of the others introduced during this sequence.
Fossa appears to know Serval through rumors she's heard by traveling to the Savanna Area. She asks her if she is "Serval, troublemaker of the Savanna area", a title which Serval denies adamantly.