Metarook (メタルーク,Metarook?) is the sub-boss of the "Destroy Train" mission in Mega Man Zero. Zero has to knock it over to strike its weak point, which is supposedly its belly. It attacks by swinging its flail and shooting from the center.
Spearook (スピルーク,Spearook?) is a the sub-boss of Tretista Kelverian's stage in Mega Man Zero 3. It's faster than Metarook, and is equipped with two spears instead of a flail and shield. Spearook fires green boomerang energy shots instead of normal bullets, and the fight against is more perilous, since it can knock Zero over the edge of the elevator during the fight. However, it is extremely easy to beat if Childre Inarabitta's Throw Blade ability has been obtained, used along with the Ice Chip, causing Spearook to tip over and freeze, allowing continuous attacks while it's rendered immobile.
In the sketches of the book Mega Man Zero Official Complete Works, there is an unused enemy called Metaknight (メタナイト) that is based in the Metarook. It has a horse head and its arms can turn between spears and maces.
In the Kirby series created by HAL Laboratory inc. there is an ally called Meta Knight. Whether or not the naming was intentional or coincidental is debated, but Capcom may have scrapped this idea because of fear of being sued by HAL Laboratory.