First of all, the rock wyverns are the Basarios and the Gravios, right?
And second is...
Have you heard about the rocks Granite and Basalt? They are both rocks.
What rock wyvern name can you form in the word Basalt? Of course, it's Basarios.
And what rock wyvern name can you form in the word Granite? Gravios.
So, Gravios came from the word Granite which is a rock. Same for the Basarios, it came from the word Basalt which is a rock, too.
Anyways, this is just my OPINION and might be true...