Skywhales

Skywhales are giant dolphin relatives that evolved flight by replacing their fins with giant wings, growing lightweight, swapping fish for birds and bug swarms, and evolving very lightweight bones like birds.

They are ubiquitous in the Sandbox, seen everywhere from Subsolar Sands to the Frozen Wastes, there are 41 species, including a domesticated one used to fly around before the invention of the airplane.