While the idea of Season Passes is to offer players all the upcoming DLC for a title for a discounted rate, it seems Evolve won't offer everything under that one-cost umbrella.
Shacknews reports there is a lot of DLC that's not included in the Season Pass, exampling around forty items - mostly skins for hunters and monsters - on the Xbox Store that aren't included in the Pass. The total price comes up to around 130 dollars.