Grrrol did not originate from fanon, and the information on this page may not be entirely accurate to their canon. You may read more about this subject in the context of their canon on the following wiki(s):
Grrrols are round, stone enemies that appear in the Mario series, and made their first appearance in New Super Mario Bros. U. They appear to be similar to Bristles from Paper Mario: The Thousand-Year Door and Spiky Tromps and Spiny Tromps from Super Paper Mario. To attack Mario, they roll across the ground and crush him, bouncing off walls, themselves, and other solid objects. They are made out of stone, similar to Thwomps, Whomps, and Wallops. Like said enemies, they are invincible to any attack save for Invincible Mario or hitting a block from below, which can defeat them.
Larger Grrrols, called Mega Grrrols, also exist, which act the same as regular Grrrols. They are, however, larger and harder to avoid.