It may very well have been a piece of the fountain come to life, but it was supposed to be a harpy, a half-bird/half-woman creature from greco-roman mythology.
According to legend, Zeus became angry at the prophet Phineas for sharing too much secret knowledge with mankind. He imprisoned Phineas on an island infested with harpies. Every day, a sumptuous feast would be laid out for Phineas, but whenever he tried to eat, the harpies would swoop in and eat nearly all the food leaving just enough for Phineas to survive.
Jason and his Argonauts showed up and killed all the harpies in exchange for information from Phineas.