The following article is based on a subject that has not been officially named in any official Square Enix material; the current title is merely a placeholder.
A guardian beast!
It must be protecting the Crystal.
Guardian Beast (守護獣, Shugo Jū?) is an enemy family in Final Fantasy Dimensions.
The Watchbeast and Protector enemies both share the same name in the Japanese version, both using the same kanji as given for the name of this enemy family. The term "guardian beast" is used in dialogue where the party point out that the guardian beast is protecting the crystal.