They were Argonauts and played a particularly vital role in the rescue of Phineas from the harpies. They succeeded in driving the monsters away but did not kill them, at a request from the goddess of the rainbow, Iris, who promised that Phineas would not be bothered by the harpies again. As thanks, Phineas told the Argonauts how to pass the Symplegades.
Apollonius Rhodius. Argonautica I, 211-223.