I might not know much about mythology but here are some basic mythological creatures

BANSHEE: a banshee is a female spirit in a Irish forklore who heralds the death of a family member usually by wailing, shreiking, or keening.

MEDUSA: in greek mythology, medusa also called as gorgo, was one of the three monstrous gorgons, generally discribed as a winged human females with living venemous snakes in place of hair, those who gazed into her eyes would turn to stone.

NUCKELAVEE: nuckelavee is a horse like demon from orcadian mythologythat combines equine and human elements it has its origins in Norse mythology.

DULLAHAN: the dullahan also called gan ceann, is a type of mythological creature in Irish forklore. He is depicted as a headless rider, on a black horse, who carries his own head held high in his hand.