Yoichi is an unseen character mentioned by once in Shin Gaiden (Benizakura's story).
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.
He said his four puppies were much bigger than Chako's puppies.