Aithusa is a white dragon that hatched by Merlin saying its newborn name.
Biology
Aithusa is a small white dragon with big blue eyes, with only thin bones and a near-transparent membrane for his wings. Aithusa's gender is not clearly stated yet, but Kilgharrah referred to Aithusa as a "he" ("A white dragon is indeed a rare thing... and fitting, for in the dragon tongue you named him after the Light of the Sun...").
History
Having spent twenty years hunting for the three parts of the Triskelion that would lead him to the tomb where the dragon egg that would become Aithusa was kept, Julius Borden sought to control the dragon to gain power. Although he offered to share that power with Merlin when the two discovered the egg in the tomb, Merlin rejected the offer and fled with the egg as the tomb collapsed due to security traps that were activated after the egg was removed from its pedastal.
Merlin later summoned Kilgharrah to learn how to hatch it, and was informed that dragon eggs would only hatch when given their names by a Dragonlord. Using this power, Merlin summoned Aithusa into existence, the small dragon subsequently hatching from the egg. Kilgharrah noted that Merlin's chosen name was very apt as it was the dragon term for the light of the sun, reflecting how Aithusa- a rare white dragon- would represent a new dawn for the world that Merlin and Arthur would create together.
legends
In the legends, Aithusa killed the welsh red dragon after a long and gory duel.