They are a beef and chicken blend and they are not very tasty.
They are same same throughout WDW at every Disney-run food service. Don't expect much with the buns, either. For such a big place you'd think there would be some variety, but the folks who run Disney dining don't see it that way. I guess it doesn't matter so much when you pile a bunch of crap on the hot dogs, but I eat mine plain and the taste of the actual hot dog really matters. One of the latest creations at Caseys is the "Chicago-style" hot dog, which is on a poppy seed bun and has a tone of crap piled on it.
I've been on a personal quest to find all-beef hot dogs at WDW for months and so far the only place I've found them is at Wetzel's Pretzels on the boardwalk at Downtown Disney, and they come in a pretzel wrap, not a bun. It's beyond sad when you can no longer get a simple, traditional American hot dog at WDW.