They're boasting about race records

 Some race horses staying in a stable. One of them starts to boast about his track record. "In the last 15 races, I've won 8 of them!"

Another horse breaks in, "Well in the last 27 races, I've won 19!!"

"Oh that's good, but in the last 36 races, I've won 28!", says another, flicking his tail.

At this point, they notice that a greyhound dog has been sitting there listening. "I don't mean to boast," says the greyhound, "but in my last 90 races, I've won 88 of them!"

The horses are clearly amazed. "Wow!" says one, after a hushed silence. "A talking dog."