Mountspecial can be used as a slash command (/Mountspecial ) causing your mount or druid 
[Flight Form] to perform a special stunt, depending on the mount.
For ground mounts, this animation can be triggered while standing and pressing the space bar, but for shapeshifted forms and flying mounts, only the slash command can be used. As of Patch 2.2, this function cannot be used anymore while moving. More recently, the command has been 'fixed' to only work while at altitude zero.
List of stunts per mount type
- Rams, Horses/Deer and similar mounts, Elekks, Hippogryphs and Mammoths: rear up.
- Sabers, Silithids, Wyverns and Cat Form/Travel Form: throw back head and roar
- Kodos, Wolves and Bears: throw the head from side to side with a roar
- Raptors, Mechanostriders, Hawkstriders, Raven Lord: stretch head upwards, all except Mechanostriders roar/squawk. Hawkstriders and Raven Lord also flap/spread wings
- Turtles: hide in shell
- Nether Rays: move in a wavy motion
- Dragons/Drakes: spread wings and roar
- Proto-drakes: flap wings, rear up and roar
- Flight Forms: perform a loop
Note: though some animations imply the mount makes a noise - for example, "roars" - only a few make a noise.
Macro to make this command more readily usable
To make the command more "on the go", you can try making a macro like this:
/script UIErrorsFrame:UnregisterEvent("UI_ERROR_MESSAGE");
/mountspecial
/script UIErrorsFrame:RegisterEvent("UI_ERROR_MESSAGE");
and then mapping it to ctrl-space, which normally is unoccupied. Unhooking the errors is done so you don't get errors if you hit the buttons while moving.