Cerberi is a unique word that refers to a mythical creature from Greek mythology, specifically the three-headed dog that guards the gates of the underworld. While there may not be too many direct synonyms for this unique term, there are several other words that could be used to describe or reference the same concept. For example, one might use the word "hellhound" to refer to Cerberus, as it captures the same idea of a fearsome, supernatural animal that guards the realms of the dead. Other similar words might include "guardian," "watcher," or "protector," all of which capture some aspect of the creature's role in mythology.