Was Griever created by Squall's mind?
Most probably, yes.
Here's Ultimecia's original script in Japanese:

アルティミシア
「おまえの思う、最も強い者を召喚してやろう」
「おまえが強く思えば思うほど、」
「それは、おまえを苦しめるだろう」
「ふふっ」

translated to:

Ultimecia
&quot;Let me summon the one you think is the strongest thing.&quot;
&quot;The stronger you think it is,&quot;
&quot;the more it will torment you.&quot;
&quot;Heh heh.&quot;

This is because Squall admired and believed Lion as a powerful creature, as described in Final Fantasy 20th Anniversary Ultimania File 2: Scenario guide book, page 247:


Image courtesy of Squall_of_Seed

Griever
The legendary lion famous as king of the beasts
A name attached to Lion, an imaginary animal that doesn't exist in this world. Squall, who admires its solitary figure, had his gunblade and accessories etched with lion design, and made it as a foundation in his closed mind.
⇐ The sorceress Ultimecia materialized Griever from images in Squall's mind as a G.F. and used it in the battle.