The Gwythiant Fledgling that Taran befriended is a baby gwythiant. Taran found him near the edge of a forest, and took care of him. The gwythiant did not seem to like King Fflewddur Fflam very much, and Eilonwy said once that "That gwythiant can sense that you don't trust him, Fflewddur!"
Taran put the fledgling in a cage, and one night, as the band slept, the gwythiant escaped, just as Doli said he would. He was then seen again when he came across Gwydion and revealed to him much of what had gone on.
The last time we saw the Gwythaint, he saved Taran from falling off Mount Dragon to his death. And then repayed his debt in full by flinging himself against the Cauldron-Born in an effort to buy Taran the time he needed to escape. This was his last act as he fell under the deathless warriors attack.