Though the plots in The Simpsons tend to be biassed towards Homer Simpson and his dysfunctional family, the series has a wide variety of interesting characters. A decent portion of these characters are animals. A few of them, like Santa's Little Helper, appear consistently while the majority only pop up whenever they are needed.
Unlike those seen in Family Guy, the animals in The Simpsons are rarely characterized by anthropomorphism, which brings about a sense of realism. Not acting like humans also doesn't make the animals in the animated sitcom any less interesting. Their interactions with humans make for endless comical moments.
Plopper
Homer has lots of fun moments with Plopper, even making him crawl on walls like Spider-Man as he sings a remixed version of the Spider-Man animated series theme song. Plopper, aka "Spider Pig." (according to Marge), first appears in the movie before transitioning to the series. He finds himself in the Simpson home after Homer rescues him from getting slaughtered.
Plopper is special because of the tight bond he has with Homer. For a moment, Homer even forgets that Marge exists. Spider-Pig is hardworking too. He makes himself useful by taking up several jobs around the city, including working as a truffle finder for Luigi. Thanks to his drive, he ends up in the Springfield Wall Of Fame.
Blinky
The villainous actions of Mr. Burns always result in serious consequences and that's what happens in Blinky's case. Blinky mutates and becomes a three-eyed fish after Springfield Nuclear Power Plant owner Mr. Burns dumps toxic waste from the plant into the river. He soon becomes a subject of discussion on news items, leading to a backlash against the power plant boss.
Symbolically, Blinky is a medium introduced in the animated sitcom to ensure justice is served. Mr. Burns normally gets away with a lot of things but through one reckless action, the billionaire gets to pay for his actions. He proves a hero by destroying Mr. Burns' gubernatorial ambitions. The much deserving Mary Bailey thus gets to be governor.
Stampy
Bart loves his pets and over the years, he has had a couple of odd ones. One of them is the African elephant named Stampy. After participating in a competition on KBBL Radio's "Bill and Marty Morning Show," Bart rejects the cash prize of $10, 000 and opts to take the elephant as a reward instead.
Stampy adds to the Simpsons' financial woes and manages to entertain while doing so. The show has frequently made fun of Homer's low IQ and his decision to allow an elephant in his home reemphasizes his lack of intelligence. The elephant consumes food at an alarming rate and destroys most of the walls, leaving them in need of costly repair.
It's easy to hate Stampy for stressing out the Simpsons but destruction isn't the only thing he is about. He has a heroic and badass side too. He once breaks a dome that forms around Springfield.
Nibbles
When Seymour Skinner gets trapped during a deadly snowstorm, he gives Nibbles a note written "Send Help!" to deliver to whoever can offer assistance. Nibbles successfully delivers the note to Homer and Ned Flanders. The hamster also has a stint at NASA where he gets appointed as the designated pilot of a rocket.
The hamster's bravery makes him easy to cheer for. If not for his efforts, Skinner would be dead. He is easy to relate to, thanks to his numerous humane moments, like when he bids his wife goodbye before a space trip. Nibbles also gets a human-like funeral at the school auditorium when he dies.
Poochie
As part of the "show inside a show" segments," Poochie makes appearances in the popular Springfield TV show, The Itchy & Scratchy Show. He is a rapper dog with an attitude. Sadly, viewers aren't so fond of him. Homer is eventually replaced as the voice actor for Poochie and as a result, audiences become more receptive to the dog.
Homer is hardly good at anything and Poochie serves to remind viewers of this fact. In addition to that, Poochie is so annoying that he is impossible to forget.
She Biscuit
She Biscuit is introduced after Santa's Little Helper becomes agitated and bites Marge in Season 31. The Simpsons try to find out the reason for his behavior. Meanwhile, a loving Bart insists on sleeping next to him till he gets better. The Simpsons soon realize that Santa's Little Helper misses his mom so they work hard to reunite them.
She Biscuit is the portrait of a perfect mother. When she is reunited with Santa's Little Helper, the love she shows her son makes Marge reevaluate her own parenting abilities. She gets bonus points for bringing much-needed happiness to Santa's Little Helper's life. The Simpson family dog normally interacts with humans only.
Mojo The Helper Monkey
Homer has been known to desire what other have and after spotting Apu with a pet monkey, he decides to get one too. When the monkey arrives, Homer names him Mojo. In the episodes that follow, Homer orders the monkey to burglarize snack shops and bring back donuts. The two make a habit of eating too many donuts that they become obese.
Mojo is interesting because he becomes a behavioral clone of Homer. He sleeps and eats just like the lead character. He also sits around all day. Though he is making Homer worse, there is sadness both on the sides of viewers and the Simpson family members when a decision is made to return him.
Princess
A couple of horses have been featured in the series but Princess stands out the most due to her significance. Eager to win Lisa's love, Homer buys her a beautiful horse.T Tono one's surprise, Homer never factored in the cost of maintaining the horse. The financial burden forces him to start working the night shift at the convenience store Kwik-E-Mart.
Princess enables the series to explore the father-daughter relationship between Homer and Lisa. Homer doesn't like working but he takes a second job anyway, to make sure his daughter stays happy. Through Princess, Homer is somehow attempting to buy his daughter's love, which is a bad move. Lisa becomes too attached to Princess and when Homer is forced to give her up, the friction resumes.
Snowball I
Though Snowball I died before the start of the series (during a hit-and-run incident involving Clovis Quimby), she is often seen in flashbacks and running gags about dead Springfielders. The Simpsons have tried replacing her with Snowball II, Snowball III, Snowball IV, and Snowball V but none have managed to fill the void.
Snowball is an important animal character because, through her, viewers get to learn what life was like for the Simpsons' in the past. Through her, the Simpsons past stories have been shared, such as the time Homer wanted to rob a bank and Snowball had to stop him. It has been explained that Snowball is good enough to go to heaven but she has been denied the chance because heaven is only reserved for humans.
Santa's Little Helper
Santa's Little Helper is the only Springfield animal that has elements of anthropomorphism as he speaks English once in a while and thinks like a human. Despite being able to talk, he isn't outspoken as he prefers to act like a normal dog rather than one with special abilities.
The Greyhound's resilience is greater than that of the Simpson family members. For example, he always insists on racing even though he always finishes last. Due to his last place finishes, he has earned the nickname "No. 8." Santa's Little Helper is also more courageous than Homer, having rescued Simpson family members from danger a couple of times. Even better, he has a buddy bond with Homer, with the two often seen doing bro-activities together.