So this happened:

Since Rebels is canon, this means that the old republic cruisers are canon too. Though referred to in the clip as 'transports', they have made their way into another trailer as being in combat.

I'm kind of happy about that.

I still think those are Class VI transports which were modified versions of the Praetorian class Frigates which predated the Hammerheads.

The Databank calls it "Hammerhead Corvette"

http://www.starwars.com/databank/hammerhead-corvette

Yeah and Its armament is lousy for its size even for a transport much less a corvette.