[@VitaVitaAR] Medusa needs a separate NP to Ride and control Pegasus. She has to use Bellerophon specifically to control pegasus. Something she can't do otherwise. It's a combination of Pegasus being born from her blood, which she recreates, and then uses Bellerophon to ride. Otherwise she'd just summon Pegasus, and then he'd fly away because she has no control of him otherwise. 

Sinbad neither has the means to summon the Leviathan, Roc, or Elephant King, and neither does he have anything that would allow him to control him.