Marcel was Ross' pet Capuchin monkey on Friends Season 1. He first showed up as Ross' date for Monica's New Year’s Eve Party. Later, he ran off while Rachel was watching him, leading the the revelation that Marcel was an illegal exotic pet. Despite loving his little monkey, Marcel had to be sent to the San Diego Zoo when he reached sexual maturity and began humping every thing in sight.
Turns out that in real life, Marcel was actually a female named Katie. Katie had several other Hollywood roles including 30 Rock, the movie Bruce Almighty and she's also been spotted with Ariana Grande and Kendall Jenner. Not bad for a monkey.
The ultimate back-and-forth couple... who we all knew would end up forth in the end. But it took a baby and many other boyfriends and girlfriends in between before Ross and Rachel arrived at that same conclusion on Friends. Just never use the word "break" around these two.