We don't have synonyms for grullo yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are grillo, trullo and grulla.
(n) :
A colouring of horses characterized by smoky or mouse-colored hairs on the body, often with shoulder and dorsal stripes and black barring on the lower legs.
(n) :
A colouring of horses characterized by smoky or mouse-colored hairs on the body, often with shoulder and dorsal stripes and black barring on the lower legs.