Salazar believed he was the only Dragon-Pony Hybrid and lone survivor of the Eldrince Empire. And this is all because of an extermination campaign that Tomir Sena of the Selenium Empire carried out himself. Not much is known about Salazar's past, but his family were allies with the Selenium Ponies making his hate for them go off the charts. After Hyros became king, Salazar secretely joined Zara and Metayel due to their share of hatred towards Tomir.
After Salazar found out that the Eldrince Empire just went into hiding, he started to believe that his own people decieved him.