Whatever comes to mind...

[Geocorona]

I have a sketch I want to turn into a monster. What kind of monster would you see this thing being?

 

[DnDChick]

Definately an Aberration if its from the Material Plane.

If its from somewhere else, an Outsider for sure.

 

[BOZ]

it looks all astral-ly.

they remind me of mind flayers for some reason - perhaps either an ally or an enemy.

 

[Krishnath]

Looks like something from the far realms of insanity where the Great Old Ones dwell...

In other words, an outsider.

 

[Geocorona]

It does have that creepyotherworldliness. Maybe it could be an OGC substitute for the mind flayers, though I don't think it should be subterranean or eat brains.