WASD space ctrl
W pitch Down
A roll/turn left
S pitch up
D roll/turn right

space (or the button mapped to jump)
ctrl (or the button mapped to crouch if there was such ability)
: space & ctrl will allow you to change height vertical only, since dragons can hover locally.

mouse-movement: aiming, within the limits of the dragons neck, auto-aim would be enabled, since It would really hard to fly and aim at the same time; the mouse aiming would only be there to select targets, so you'll be able to choose your target if there are several.

Your flying mount would be able to perfectly gain height without the space and decrease heigth vise versa, no GTA flying controls wich are quite hard to handle. Even with two hands.

It's one of these days...