Varies greatly based on fandom, each “category” has their own specific tags. If you can stand/like blood, scroll through the “vampire au” tag filtering in the fandom you want, then see if the big authors have a tag they use. The good news is that authors who write one, generally write/bookmark more. (Other tags are wing au, mer au, etc., a general tag might be “animal instincts” but I haven’t seen that one tagged every time like “____ *character name*” is.)