The term 'werebeast' (and its shortened counterpart 'were') is an old community term that is short for werewolf or any were-creature (such as a werecat, werebear, or werebat). If someone said “I am a were”, it would have been the old equivalent of someone saying "I am a therian". This term was used by the members of alt.horror.werewolves (AHWW) since the beginning of the community in 1993, and was used up until the term 'therian' became more popular around the end of the 90s/very early 2000s.
The term is now considered "outdated", but it is still sometimes used by older members of the community, or those who believe their identity leans more towards that of a were-creature.