Am I just unlucky or does it only spawn once? I accidently killed the first One unfortunately and can’t get a second to reappear. In the sticky thread t doesn’t say this monster is rare so am I just unlucky or is it a 1/80 chance ark?
Well I found one again but it was rank d 
Skullrex has a low spawn chance. Don’t worry he does come around in the game more than once! 
He’s just rare.
Moving to FAQ - this is not asking for Skullrex’s location.
Where is skullrex found?
He’s a rare encounter in Ryodou Cave and on the northern half of the crescent island
The obserker, the answer to your question is a post above yours…
I suggest above Crescent Island. It may have about the same spawn rate as the cave, but it also has monsters such as Remus, Gargoyles, and others, which can both be used for quick level grinding and fusion Arkadians. While searching for a Skullrex, I found several S ranks and quickly got a Level 99 Skywolf. I now have a S-rank Plasmorex. You also have a 100% chance of catching monsters on the route between role named argramis, where Plasmos spawn at good rates, making it easy to get an S-rank for fusion. A-rank Skullrex and S-rank Plasmodious=S Rank Plasorex.