poor spike, he was degraded to the lower tier of the cartoon chain: "the dog" so sad and yet I couldn't stop laughing at his terrible fate hah...okay I''m done. anyway! if you haven't seen the movie trailer you can find it here: -----> [link]
I think Spike being a dog makes sense because dogs are devoted by nature , and spike was devoted to go after Twilight after princess celestia specifically told everypony else twilight had to go alone . And you have to have some really big balls to disobey someone who can control the rising and setting of the sun and moon !