Revealed previously in toy form, Zeb is Star Wars Rebels' muscle. He's an all-new alien race based on an old Ralph McQuarrie design, and as revealed in the video below at IGN, he's voiced by voice acting legend Steve Blum.
If you've watched an action anime or any animation or game with Wolverine in it in the last 15 years, you've heard Blum's voice before. He has also voiced characters like Spike Spiegel, Starscream, Devil Dinosaur, Darkseid, Electrocutioner, Lex Luthor, Rocket Raccoon, Urdnot Grunt, and many, many more.
Zeb is described in the video as the muscle of the team, but also as something of a mentor and companion to the squad's youngest member, Ezra. Stay tuned for what promises to be another week of Rebels news.