You just bought a horse off the track. First of all, thank you! These horses are amazing athletes and deserve a shot at a second career. I love seeing them being given that opportunity.
So now comes that transition period. In the first few weeks/months of being off the track it is easy for them to go from looking like this horse:
A.

To looking like this horse:
B. |
Nasty grass hay belly and coat. Loss of muscle over the top line, on the hips, and on the neck. Needs wormed. |
B. |
Shows the loss of the muscling over the top line and the long coat better. |