Hi everyone, now I just found out that this 2002 film roughly base on the 1993 film The Silver Brumby, which the latter is base on the namesake novel. So, what you're about to read is whether on not Spirit is based on the novel.

It's based on the Film

So, I can understand that it is a remake of the 1993 film, but more targeting for family oriented theme. The only difference is that the story for The Silver Brumby is set in Australia.
The horse befriends an unknown character, simply known as The Man/ Egan and is played by Russell Crowe. Just like how Spirit befriends Little Creek, a Native American, together they travel all parts of the USA and Australia respectively. Enjoying the landscapes of both countries!
Finally, they found their own happiness in the end.

It's not based on the Film

As mentioned, but stories are set in different locations, but the motive for both Spirit and Thowra are slightly different on how to stay free.
Both horses are of different horse breeds, Spirit is a Kiger Mutang while Thowra is an Australian Brumby. The latter is more mature than former for the majority of their respective films.

What Do You Think?

So, here are my opinions and thoughts on both films. What do you think? If you haven't seen The Silver Brumby, go ahead and see it! The full movie is on Youtube now.