“This form was never seen in the storyline, but is simply one of the many that Makuta could take.”
or
“This model was used to represent Graalok the Ash Bear in the BIONICLE: Maze of Shadows video game, as that character had no official model of its own.”
So, it could be either one of those, really. Most likely the former.
It would have been cool to see more of the makutas’ shapeshifting in action. We know Miserix liked dragons, maybe Teridax had an affinity for ash bears.
“Makuta Nui: One of Teridax’s many forms resembled a giant beast. This form was never seen in the storyline, but was simply one of the many forms Teridax could take.”
“Teridax’s “Makuta Nui” form did not appear in storyline. However, it was utilized in BIONICLE: Maze of Shadows to represent an Ash Bear enemy.”