Ostrich! Stuart favored black ostrich plumes. Time is not good to feathers, so it really is hard to tell what flying fauna that was plucked from.
Besides the cavalier, French musketeer, dashing image, there was a purpose to the 'feather in the cap'. Gave the men something to follow they could see through the gun smoke - the more color the better - and it also helped them co-ordinate the battlefield. The Confederates clung to that model for a long time but the Union decided the men could follow a colorful flag just as well! Not many peacocks on their side. I've always noticed the Southerners tended to look for the commander more than the flags.