One other ready-made source for monsters besides mythology is their modern equivalent, folklore beasts (AKA cryptids). TFT already includes some of those, such as the sasquatch and chupacabra. There are a number of other ones which could make good additions as well - jackalope/wolpertinger, shuck, ahool, sea monsters/serpents like Nessie and Champ, the Mothman, and the Jersey Devil. Here are some links if you're interested:
https://cryptidz.fandom.com/wiki/List_of_Cryptids
https://mentalfloss.com/article/5894...ight-not-exist