Many people wondered why Balto was never adopted. The ending of the first movie would suggest that after rescuing Rosie and becoming a mate with her dog, he would be adopted by Rosie's familly. However, everything in the second and third movie still suggest he's still a stray.

1. He still lives on the boat while Jenna lives in a house. (both sequels)

2. In Wolf Quest, Jenna mentions something about having to go somewhere with her familly. If Balto was adopted by them, he would go with Jenna.

Now what about the third movie? In the opening scene we see Jenna, Balto and Rosie spending time together. Anyways, later in the movie it's still seen that Balto and Jenna live separetely.

But then, many people believe that Balto was in the end adopted by Duke. It could be just that Balto and Duke became closer friends, as Balto is a friendly and well-known dog in Nome, but I think it's true that he got adopted. Just look at this scene:

Have you noticed Balto's collar? He never wears any kind of collar except when he's in the dog sled. I think that movie creators wanted to suggest us that Balto was indeed adopted by Duke!