This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Tritons (Greek)
Tritons were a group of fish-tailed sea-gods or daimones in the train of the god Poseidon. They were a plurification of the god Triton and were portrayed as the Satyroi (Satyrs) of the sea. Another breed of Tritones, named Ikhthyokentauroi (Sea-Centaurs), had the upper bodies of men and the lower bodies of Hippokampoi (fish-tailed horses).
They are usually represented as a merman, with the upper body of a human and the tailed lower body of a fish. At some time during the Greek and Roman era, Triton(s) became a generic term for a merman (mermen) in art and literature. In English literature, Triton is portrayed as the messenger or herald for the god Poseidon.