Alpha Chi Omega
These creatures, bred in the back of Fred and George's shop in Diagon Alley, were so popular that the Weasley brothers had difficult breeding them to keep up with the demand. Because of this reason, and because of Alpha Chi's musical heritage (Luna Lovegood believed these creatures sang on Boxing Day), their mascot would be a Pygmy Puff.
Alpha Delta Pi
As ADPi's value strength and tradition, the Centaur would be their mascot. They are proud of their species but are also known to display utmost loyalty even among wizard-kind, such as when Firenze saved Harry from Lord Voldemort in the Forbidden Forest.
Alpha Gamma Delta
The squirrel transitions gracefully from season to season, just as the Phoenix transitions gracefully from one life to the next. This bird would be the mascot for AGD because of its ease through life and its ability to progress. Dumbledore's own phoenix, Fawkes, saves Harry from the basilisk, representing a figure of true character.
Alpha Phi
Just as the ivy serves as a symbol for growth, one of my favorite Harry Potter creatures is actually considered a plant, but these plants are far from silent. Like Alpha Phi's, Mandrakes are loud and resilient. Though hard to control sometimes (just ask Ron), they bud into beautiful plants.
Chi Omega
There should be no reason to change a mascot if the Harry Potter equivalent is fitting. The Owl serves as the Chi O mascot in reality and in the Harry Potter world. Not only are they a symbol of wisdom, but the owls in Harry Potter come in many sorts of personalities and with distinct features. 
Delta Delta Delta
Keeping in with the marine theme, the Tri Delt mascot would be Merpeople. The Merpeople in Harry Potter held a mysterious, yet orderly society in which a strong sense of leadership and resilience was demonstrated when they captured Harry in the Black Lake.
Delta Gamma
Let's take the Hannah doll and amp her up; we get the mystical creature from Harry Potter, the Veela. These beautiful semi-humans have the ability to charm but are more than able to stand up for themselves. Fleur Delacour is only a quarter-Veela, if that tells you anything about their beauty. 
Delta Zeta
The turtle simply does not do these girls justice. The Hippogriff would be the mascot for DZ because of their sense of power and pride, possessing a strong demand for respect. Though birds of prey, they are simply elegant creatures. 
Kappa Delta
One of the most prized creatures in the world of Harry Potter, the Unicorn would be the mascot for KD. These beautiful creatures are so prized, however that they often remain elusive. This is because their blood will keep you alive, even within an inch of death. But killing something so graceful — so pure — is an act one pays only at a terrible price.
Kappa Kappa Gamma
The mascot of Kappa would be the Boggart, not because of its fear-inducing capabilities, but because it is like a key into one's own soul. It is omniscient, and can take on any form. It is the form of wisdom, the form of dignity. It takes on the shape of a stately woman, as well as a graceful woman. Truth, honesty and sincerity: KKG's mascot would have to represent them all. 
Phi Mu
Hear me out: the House-Elf would be the perfect mascot for Phi Mu. They are docile and forgiving. Sweet and willing to serve. But don't let these House-Elves fool you. As apparent through Dobby's exertion of his own freewill, they are highly intelligent and incomparably loyal.
Pi Beta Phi
These Pi Phi's would have no other mascot other than a Fairy. Slightly but rightfully vain due to their good-looks, fairies cannot speak but are able to laugh; what else would a Pi Phi need?!
Sigma Sigma Sigma
Hagrid's Three-Headed Dog would be the prime mascot for Tri Sig (and not just because of threes). As each head of the dog is an individual, together they become complete. Fluffy represents their open motto "Faithful Until Death" as it guarded the Sorcerer's Stone.
Zeta Tau Alpha
As ZTA's royalty holds through the crown, their mascot needs to be a top-of-the-foodchain creature: the Dragon. May it be the venomous Peruvian Vipertooth or the dangerous Hungarian Horntail, they come in many varieties with a wide arrange of talents. ZTAs may not be terrifying like the Dragon, but rather awe-inspiring. As dragons aim to protect their eggs, Zeta focuses on conserving their tradition. 
