Ginga Densetsu Riki(2000)

Riki, the grandfather of Weed and father of Gin, was born a runt. He was fathered by Shiro and mothered by Yamabuki. Shiro's owner, Gohei, pays no attention to Riki when he comes to pick a pup from the litter. Instead, Riki is never rehomed and becomes close to his mother. Unfortunately, Yamabuki's true owner, who had been sick, was healthy enough to take her back. She didn't feel she could handle two dogs, and decided that he would not take Riki in. Riki severely missed his mother, and one day set out to meet her. On his way, he is attacked by a pack of dogs. Hopelessly outnumbered, Riki tries to escape and falls into a river. Waking up, he learns that his father, Shiro, has rescued him. Riki had never seen his father, but had known his name. Shiro, on the other hand, didn't know that Riki was his son and left before Riki could say anything. But, not before giving him a speech on being strong. Riki makes it his goal and constantly tries to get stronger.

(Source: Wikipedia)