Most sitcoms don't include a whole lot of pets - most likely for purely practical reasons, as animals are notoriously hard to deal with on set. However, Friends isn't most sitcoms, and over the years, a whole host of pets have joined the show, whether long-term (like Marcel, who was Ross's bizarre choice of companion for a long time) or just for an episode, or even a single scene.
But of all the pets that made an appearance on Friends, which are the ones that had the biggest impact? Which were the cutest, the most beloved, the most iconic of the animals with the best storylines... and which have fans probably already forgotten about?
Mitzy The Hamster
Mitzy the hamster is never actually seen, but that's only because her owner's apartment is so appallingly messy! Mitzy appears in 'The One With The Dirty Girl', when Ross starts dating a wonderful, beautiful, woman... who lives in filth. When he goes to her apartment, there isn't just mess, but what seems to be leftover food in between the couch cushions, and a free-range hamster that the 'dirty girl' tosses food on the floor for. Fans feel for Mitzy, but as the hamster is only ever mentioned in one episode, Mitzy gets the bottom spot.
Frank Jr's Dog, Schnoodle
Next up is another one-and-done pet, Schnoodle. This annoying pup appears in 'The One With The Bullies', where Phoebe goes to find her biological father, but ends up stuck in the cab, because every time she tries to get out, Schnoodle (a terrier) starts leaping up, barking, and attacking the window. Phoebe accidentally ends up running over the dog, too, and takes it to the vet before bringing it home and learning her father doesn't live there... but her half-brother does. Points for introductions to Frank, but Schnoodle was just annoying.
Paolo's Cat
This cat is absolutely adorable, but essentially inconsequential, with only one appearance. In 'The One With The Blackout', Rachel and Ross end up finding this furball (or rather, it attacks Ross on the balcony), and then Rachel decides to find its owner... which turns out to be Paolo. Of course, Paolo is one of the most hated characters on the show, a total sleazeball who hits on Phoebe while dating Rachel, but at least he has a cute cat.
Eddie's Goldfish Buddy
Buddy gets points for being part of such a ridiculously funny plotline, although the goldfish himself isn't that interesting - after all, he's a simple goldfish! However, Eddie (Chandler's crazy temporary roommate) accuses Chandler of sleeping with his ex and then killing his fish, before replacing the original fish with a goldfish cracker in a tank.
Chappy
Mike's family dog, Chappy, appears in the episode where he and Phoebe get married. He's not the most adorable dog, and is actually absolutely ancient and fairly sickly, but he's a great addition to the show because of the plotline... which sees Ross losing out in the end, because he's childishly trying to use Chappy to be part of the wedding party. In the end, he is stuck holding a smelly dog, and waiting outside for it to pee, while the others get big parts in the ceremony.
Mrs Whiskerson
Rachel has some amazing moments in Friends, but sadly, this is not one of them. In 'The One With The Ball', Rachel buys herself a purebred Sphinx cat - because her grandmother used to have one that she loved. However, the cat is not incredibly friendly, but hisses and scratches her, so she decides to try and get rid of it. (Definitely one of the worst things Rachel did, treating a pet like a disposable object.) Gunther ends up buying the cat, because he's in love with Rachel. Overall, a terrible plotline, but Mrs Whiskerson does have a decent-sized plotline and appearance in the show.
Phoebe's Rats
Phoebe is known for being eccentric, and an animal lover - so it's actually not too surprising that she decides to raise a box of baby rats. In 'The One With Phoebe's Rats', Mike ends up (intentionally) killing a rat in the apartment, and Phoebe is heartbroken about it - especially when she learns that 'Bob' had babies.
The two end up taking care of them, which is adorable on its own, but even more so because it shows how perfect Mike is for Phoebe, that he understands why she wants to, and is able to show her how it may not be the best thing to let a box of rats take over their lives.
Clunkers
In 'The One Where Chandler Doesn't Like Dogs', Phoebe has been staying with Monica and Chandler, and reveals that she has been hiding a dog in her room (temporarily, for a friend). This dog is the absolutely ridiculously adorable Clunkers, and the episode ends up with a lot of light comedic confusion over where the dog is, and if Chandler lost it, and why Chandler hates dogs. It's a bit of a silly plotline, but Clunkers makes up for it by being so cute.
The Surrogate Puppy
Another animal who gets points for cuteness, as well as story impact, is the surrogate puppy that Phoebe's birth mom loans her when she is trying to decide if she should be a surrogate for her brother. For one thing, this little pug is painfully adorable, but the storyline itself is a solid one. Phoebe struggling with the decision is important, and Friends manages to balance humor and drama when Phoebe gives the puppy to Frank Jr and Alice... without realizing that it was just a loan, and her mother wanted her puppy back!
Chick Jr & Duck Jr
The second incarnation of the Chick and the Duck are given to Monica and Chandler by Joey at the end of the series - and end up finding their way into the foosball table, which then has to be destroyed. In the end, Joey decides to keep the birds, and everyone has a sweet moment where they remember the originals, and think of all the great moments Joey and Chandler shared. They do only appear briefly at the end, though, which is why they don't rank higher.
Julio/Phoebe's Mom
In another instance of Phoebe's kookiness, a cat wanders into her guitar case, and Phoebe decides that it is actually her dead mother reincarnated. However, when Ross finds 'lost cat' posters around town, he confronts Phoebe about taking a family's cat - and things get pretty heated.
This actually ends up being a fantastic episode, though, as it covers supporting friends, even when their beliefs are different, and having empathy - as well as Phoebe dealing with her guilt over connecting with her birth mom.
Marcel
Probably the most famous pet on the series is Marcel, Ross's pet monkey. Ross gets the pet after his divorce, when he is most definitely going through some things, and brings Marcel around everywhere. At first, it's an odd quirk, and he causes some havoc with changing TV languages, playing 'The Lion Sleeps Tonight' on repeat, or in his interactions with Ross - but in the end, Ross has to give him up to San Diego Zoo because (spoiler alert) it's not appropriate or good for a monkey to be kept as an apartment pet. Marcel is definitely one of the most iconic animals on the show, but he's also one of the most ridiculous.
The World's Happiest Dog
Officially named 'Mozzarella', this adorable pup belongs to a friend of Phoebe's, and she brings him over to cheer Joey up when he's feeling miserable about Rachel. He's one of the best animals on the show, not really for the storyline (most fans hated the Rachel and Joey pairing), but because he is so incredibly cute and energetic, and every fan's heart broke when they saw how sad he was after listening to Joey pour his heart out.
The Chick and the Duck
Could there BE a more iconic pair of pets from Friends? The Chick and the Duck come into Joey and Chandler's lives after a news item on people buying chicks for Easter (and throwing them away, which is bad) encourages Joey to buy a chick of his own (because he misses the point). The two became fixtures on the show for a while, though, wandering in and out of the apartment, providing plenty of joke fodder, and generally being amazing (and surprisingly well cared for), until they are 'sent to a farm'... which was, of course, just what Chandler told Joey when they died of old age.