Slytherin is probably the most misunderstood House of them all, but just like the other Houses, Slytherins have their mascot animal.
Is it an obvious one if you think about it and it suits them rather well. It also represents water, the element associated with Slytherin House, and the color green belonging to Slytherins.
What Creature Represents Slytherin?

The Slytherin animal is a snake.
The first thing that comes to mind while thinking about the snake is that they are not very nice. It is the same notion most people have about Slytherins.
The truth is that snakes are rather clever and intelligent creatures, using many disguises and tricks to get what they want. We think that snakes are very underrated and should get more credit.
Why is Slytherin’s mascot a Snake?
Salazar Slytherin picked the snake as a House animal because he could speak Parseltongue, the snake language. That is probably the most important factor above others.
The snake also represents Slytherins rather well because it usually avoids fighting if it can help it. It is also a very cunning animal, always figuring out ways to get what it wants. Slytherins are known to weigh their options before doing anything, and just like a snake, they go for what they want, usually no matter the costs.
Slytherins are also clever folks, and although they are usually not Ravenclaw-smart, they do think a lot to make convoluted plans of getting the job done. Just like their mascot, they can wait in hiding before the strike and often let events unravel themselves.
Although most of the traits associated with the snake are perceived as unfavorable, we certainly don’t think so. The snake can be venomous, but we have to admire its skills and technique.
Slytherins had a long and troubled journey through the Harry Potter series, and they are complicated but most fascinating House at Hogwarts.
There is usually a double meaning to their actions, and while most other wizards think the Slytherins are all evil, we know that one should not paint all of them with the same brush. Just like snakes, Slytherins come in all types, the venomous and dangerous, but also tame and harmless.
In some beliefs, the snake symbolizes rebirth through losing its outer skin and immortality – the Ouroboros biting its own tail. We know that many Slytherins would want eternal glory, which equals being immortal (in our opinion, Voldemort went a little too far).
After the Wizarding War, many Slytherins also changed their skin and were reborn as better citizens in a new world. The best example is Draco Malfoy, the perfect Slytherin.
Conclusion
Snake is a great representation of Slytherin House. It usually slithers unnoticed, doing its own business, but provoked can be very aggressive and often scares others away. Slytherins, from what we learned in books, are portrayed to be the same.
Although we know that there are also tame, pet snakes/Slytherins out there, cunning and ambitious, who don’t mind helping others a little, without self-interest in mind.