It's both, actually
Terraria Community Forums
GoldenTerrabyte
Rastriavis
Ah, cool! Is it completely tied into Terraria, though?
GoldenTerrabyte
Well, it's a multipurpose character. His appearance is primarily based on how he looks in Terraria though. In most drawings of him, that is. In other games for instance, he can look different, just depending on what's possible.
Rastriavis
I have another question... What kind of dog is it?
GoldenTerrabyte
He's a werewolf, but he prefers to stay in a wolf form all the time.
GoldenTerrabyte
Is there a reason you're collecting information, or are you just curious?
Rastriavis
Just curious. 
GoldenTerrabyte
That's cool. I always love answering people's questions.
Rastriavis
Awesome. It's kinda funny, because I never really go into specifics of which species of cat and dog were merged to create my character.
GoldenTerrabyte
Yeah, I just find coming up with all the little details to be fun.
Rastriavis
I came up with all the other details, but for the life of me I can't think of an explanation of those two in particular.
GoldenTerrabyte
Yeah, I feel you. I've had similar stuff with some other characters.
Rastriavis
The thing is, I'm not that pressed to come up with the specifics, it gives me an explanation of why his siblings look different.