As for cloud terrors. Perhaps it could use the model of DnD monster Stirge: https://goo.gl/images/7MWCN4
Have it as some sort of Pterodactyl with less facial feature to indicate their short life span.
As for how rare it spawn. Quite rare, as rare as magma cube in the nether i guess? But keep in mind that this creature spawn "naturally" inside other mobs. Not outside.
As for the attack? I guess it deals 2 hearts of damage but ignores armor. So it's not THAT big of a deal to be surprised. But it's dangerous if it appears in a big flock. Though it's very unlikely.
And the original concept is that it doesn't affect the player's life. Only death.
As for name: Cloud terror is one, but there's other alternative like Sky Matriach, Cloaker, Winggidon, Levicyte.