Learn Spanish (Latin America): ¿Vas a competir con un perro suyo? - Are you going to compete with a dog of hers?

su perro translates to "her dog," which usually implies a specific dog or her only dog. Using un perro suyo means "a dog of hers" or "one of her dogs." We put the long possessive adjective (suyo) after the noun to create this indefinite meaning.

Does suyo agree with the gender of the dog or the gender of the owner?

It agrees with the dog. Possessive adjectives in Spanish always match the noun being possessed. Since perro is masculine and singular, we use suyo. If you were talking about a female dog, it would be una perra suya, regardless of whether the owner is a man or a woman.

suyo can mean "of his," "of hers," or "of theirs." How do we know it means "of hers" in this sentence?

By itself, we do not! Out of context, un perro suyo could mean a dog of his, hers, yours (formal), or theirs. We assume "of hers" here based on the English translation provided, which implies the speakers already established who they were talking about. If you needed to clarify out of the blue, you could say un perro de ella.

Could I say un su perro to mean "a dog of hers"?