I think the below, taken from Timeform US blog, is the best summation of the different running styles. Most horses, in my opinion, fall between the Speed/ Tracker/ Midpack categories. On dirt at least. Turf is a different story. Please forgive me for referencing a different product. and if it’s looked down upon, I’m happy to remove the post.

Leader (A one-dimensional front-running horse)
Speed (A horse who races on or near the early lead)
Tracker (Generally a stalking type of horse, or one with a very tractable running style)
Midpack (A horse who generally races in the middle of the pack)
Closer (A stretch-running horse who comes from the back of the pack)
Plodder (A one-dimensional horse who languishes at the back of the pack)