The term "horse's ass" is a colloquialism used to describe someone who is foolish, obnoxious, or annoying. However, it's always a good idea to have a few synonyms in your arsenal to avoid repetition. Some possible synonyms for "horse's ass" include "jerk," "idiot," "nitwit," "moron," "fool," "buffoon," "blockhead," "twerp," "dolt," and "imbecile." Additionally, you might also consider using less profane terms like "prick," "jackass," or "knucklehead." Ultimately, the synonym you choose will depend on the context and level of severity required to convey your intended message.