The Adamant Turtle is an enemy from Final Fantasy Mystic Quest.
The creatures have two phases. The first phase has them rearing their head back in a menacing manner. The second phase has them tucked into their shell.
Adamantite and similar words including Adamant and Adamantine are used to refer to any especially hard substance, whether composed of diamond, some other gemstone, or some type of metal.
In the German release, the enemy is known as "Maden Fresser" which means "grub eater".