Yoichi is an unseen character.
Since it's not mentioned clearly, he was either a man living in the village or a male Tosa dog owned by a villager.
In case he was a dog, he had four pups with a female Tosa.
In case he was a human, he owned a female Tosa which had four puppies with a Tosa dog.
It's said his four puppies were much bigger than Chako's puppies.