Season 1: Episode 19: The One Where The Monkey Gets Away
Just as Ross prepares to tell Rachel how he feels about her, he finds out that she lost Marcel while babysitting him that day. When the gang learns of the disappearance, they go on an all-out search. Rachel and Monica are surprised to find a girl they snubbed in high school working for Animal Control, and she is determined to have Marcel destroyed to get back at them. Suddenly it's a life or death situation. The gang finds him first (he's been taken in by their creepy neighbor, Mr. Heckles, who has dressed Marcel up as "Patty" and is pretending it's a different monkey), but the Animal Control woman shows up moments later. Luckily, Rachel is able to convince her to look the other way, and Ross gets to keep his monkey. Meanwhile, Ross gets over his anger at Rachel and tries again to express his feelings for her, only to be interrupted by her ex-fiancie, Barry, who tells Rachel he still loves her.