Ask yourself: what is your favorite animal?
Do you have a beloved pet, past or present, whom you want to honor? Do you have a spirit animal, or a Patronus? Does your school or a college that you like have an animal mascot? Is there an animal with a religious or spiritual meaning that you like, like a dove or a lion? What is your favorite meat--beef, lamb, pork, chicken--and can you pay homage to that animal? Do you have a favorite animal fictional character or cartoon character, like Charlotte the Spider or Bugs Bunny?
I see Halloween as an opportunity to express a part of yourself that you rarely get to show. Dressing as a favorite animal, or as an animal you like or admire, can be a fun part of that self-expression!
BTW, this year (2018), I'm dressing as the Four Living Creatures of the Bible: three animals at once, plus my own human face!
I also once dressed up as a buffalo: not for Halloween, but for a costume party at New Year's Eve, where we had a "Western" theme that year. I came in second for the costume contest!