I know therians dont have a hidden ability but database say they do.
They have hidden abilities and you can them in a game for the 3Ds called Dream Radar. You catch the Pokémon in Dream Radar and can transfer them to Black 2 or White 2.
Edit: I looked up dream radar on the eShop, and it is on there. It costs $2.99 and I would’ve bought it but I don’t have B2 or W2. So yes, they do have hidden abilities, and you can still get them.
They all do have a hidden ability. You get them through Dream Radar in gen 5, so they are currently unobtainable.