Pterodactyls are creatures that live on Mobius and prehistoric Earth. The first one seen in Sonic the Comic was a friendly pterosaur in the first Ecco the Dolphin story; it originally looked as if it was going to attack Ecco, but actually sided with the dolphin, taking him to a glyph in the sky.
Several more appeared later in the Great Mobian Prairie belonging to a group of Apterix breeders. Knuckles the Echidna was given one and it stayed with him until he eventually made it back to the Floating Island. It is unknown what happened to it after the echidna returned home.