Quote Originally Posted by Gazius
Where did the indo-greeks learn about dolphins?
First of all, there was communication with the Mediterranean world they left behind, albeit very scarce and with problems... (check for periplous in wikipedia if you want to know more), and also we have accounts of mercs claiming to have served in India.

When IndoGreeks conquered the Indus delta they became re-acquainted with the "sea" their ancestors have been raving on and on about. Hellenes had always had a special relation to the sea, because they saw it not as an obstacle but as a means to an end, a "bus stop" if you will on the way to home. We still do! (Xenophon and his 10.000 crying "Thalatta!=sea) and all that.

As for the dolphin, it must have remained in popular mythos as the faithfull companion of ships in open sea. Once reconnected the sea, they would know what it was. If they didn't the greek merchants from Alexandria coming to India, would tell them, soon enough.