"Restless" means unable to stay still or calm, "restive" means resisting control or becoming difficult to manage. Both adjectives are used frequently in the literature on training of horses.
Example usage of the former:
"he annoyed me excessively by his restless dancing and fretting" - Horsemanship for Women, by Theodore Hoe Mead, 1887
Example usage of the latter:
"It is better not to fight a restive horse" - Horsemanship for Women, by Theodore Hoe Mead, 1887, "unless you have reason to be sure of victory".
