What breed of horse are grullas or spelled grullo?

"Grullo" refers to a color, not a breed of horse. It's a specific shade of dun that has a grayish-blue body color with a black mane and tail.

Here's how it works:

* Dun is a base coat color that can be found in many different breeds.

* Grullo is a specific variation of dun characterized by the grayish-blue body color, black mane, and tail.

Therefore, a horse can be a Grullo regardless of its breed. It could be a Quarter Horse, an Arabian, a Thoroughbred, or any other breed, as long as its coat color meets the definition of grullo.