Quote Originally Posted by Grey_Wolf_c View Post
No, it is not incorrect. Source.

Grey Wolf
Top of the statblock: "Aboleth Mage, 10th-Level Wizard".

And it has the relevant extra 10d4 HD.

I agree with you that we can't just add any random class levels we want, but the Aboleth Mage is in fact an Aboleth with Wizard levels and a few arbitrary-source attribute boosts.