This dolphin (I do hope that it is a dolphin, I am by no means an expert on the subject) is to be found in Ostia. You can compare it to this ugly fellow. More on ostia is found at day 45, 46, 55, 56 and 74.
"The dolphin is an animal not only friendly to man, but a lover of music as well; he is charmed by melodious concerts,1 [p. 2372][sic] and more especially by the notes of the water-organ.2[sic] He does not dread man, as though a stranger to him, but comes to meet ships, leaps and bounds to and fro, vies with them in swiftness, and passes them even when in full sail."
Pliny the Elder - Naturalis Historia (the Natural History) IX.8
Translated by John Bostock, H.T. Riley