I'm going to start with the first clue and focus on that. First, we know that it's "Quite early". I'm going to make a wild guess and say you're referring to Dinosaurs, therefore meaning this dinosaur is most likely from the Triassic. Next, demonic. Probably meaning horrid-looking, but perhaps it simply means that it is a predatory dinosaur. Or perhaps it means Demonic Teeth. However I'm guessing it's just weird teeth. So let's say Demonic means predatory, just to try and narrow it down a bit. Coelophysis, Herrerasaurus, Staurikosaurus, Eoraptor, Tawa, Daemonosaurus, Chindesaurus, etc. But one name sticks out: Daemonosaurus. Meaning Demon Lizard and has very unique teeth. About the size of a dog. Daemonosaurus it is?