[WIP] My Little Pony FIM Fan Art

I have a brother who is madly in love with the series My Little Pony Friendship is Magic and because of that, I want to make him a halfway decent game that he can play that revolves around the ponies. I also want the practice modeling and building a game. The problem I have run into is that there is a seam that runs down the pony’s back when I turn up Subdivision Surfaces. I don’t have any idea why that seam is there other than that it must be a result of mirroring the model across the x axis. I have posted pictures below in the hopes that someone can help me fix this problem.

Remove any faces inside the mesh, especially along the mirrored axis
In mirror modifier ensure clipping and merge are enabled with a merge value sufficient to capture all vertices along the mirror line
Remove double vertices (select all vertices and W / remove doubles
Recalculate face normals (select all faces and Ctrl+N)

Thank you so much! Your suggestion worked perfectly! Now all I have to do is figure out how to unwrap and texture it.