"The guy is mean, I mean really mean!"
Tommy is a man who dates Rachel in season 3, but they break up because he is evidently filled with rage, which he does well to hide from her while in her presence. He frightens and intimidates anyone who irritates him in the slightest way by viciously screaming at them.
Tommy pretends to be a nice guy while in the presence of Rachel. However, after meeting Ross and Rachel at the theater, Ross sees his true colors after witnessing him intimidate an elderly couple who were accidentally in their seats by yelling his throat hoarse at them and calling them stupid (Rachel was absent at the time).
A stunned Ross tries to warn Rachel and the others what an aggressive bully Tommy is, but they refuse to believe him, assuming he's just making up the story about Tommy screaming at the elderly couple in order to split them up and have her all to himself so soon after Ross and Rachel broke up. Ross is also teased over this by Chandler, much to the former's chagrin. Tommy later uses the fact that Ross almost spills hot coffee on him as an excuse to intimidate him by ranting and shouting insults at him, but the rest of the friends do not see this happen. Intimidating people in this manner apparently amuses Tommy, who appears to be in a good mood after frightening both Ross and the elderly couple, but he does not let Rachel see his aggressive true colors, laughing off the fact that Ross has almost spilled hot coffee on him when he does it a second time while in her presence. Considering that Tommy had learned of Ross's past with Rachel and witnessed how awkward they were around each other prior to screaming at both the elderly couple and Ross himself, it is likely that Tommy guessed after witnessing this that she wouldn't believe Ross if he ever told her what a bully Tommy really was.
But Tommy's true colors are finally exposed to her when he loses his temper after Joey and Chandler's pet baby chicken urinates on his hand, and their pet duck irritates him by quacking at him. Unable to keep his anger in check, he takes it out on both of the animals by screaming at them - loud enough for Rachel, Monica, Phoebe, Chandler and Ross to overhear him and walk into the room. When he sees them all staring at him, and stunned, he, to his own chagrin, is forced to accept the fact that Rachel and the others are now scared of him, and breaks up with her before leaving. He is never seen again after that.
"(Ross accidentally bumps into Tommy) What's in the cup Ross? (Ross:Uhm!) What is in the cup? (Ross: Okay, it's coffee) Iced coffee? Tell me it's iced coffee. (Ross: It... It's hot...) HOT COFFEE! YOU IDIOT! YOU'RE GONNA SPILL HOT COFFEE ALL OVER ME HUH? WHAT ARE YOU? JUST A BIG DUMB STUPID DOOFY IDIOT WITH A DOOFY IDIOT HAIRDO HUH? HUH?"
"IDIOT! YOU STUPID LITTLE FUZZY YELLOW CREATURE! OH LOOK AT ME I'M SO CUTE, I'M A LITTLE CHICK WHO'S DISGUSTING! GOD YOU'RE SO STUPID, HOW ARE YOU NOT YET EXTINCT!"
"QUACK QUACK, WHAT ARE YOU QUACKING ABOUT?DUMB DONALD DODO!"
"Well I guess we're not going out anymore. MAN!"