Beet pulp is an excellent source of fat for horses, and it digests in the hind gut rather than the secum. This means that you can feed it at a high volume without the worry of it upsetting the pH of the hind gut, which other feeds (grains, complete feeds, etc) will do if they are fed at high volumes because they are digested in the secum, but if that overfills it will spill into the hind gut and disrupt it. When the hind gut pH is disrupted, you end up with founder, colic and in severe cases complete kidney failure.
So, yay beet pulp! As long as you are building up the amount slowly, you can end up giving them as much as they need for weight building and it is great for young horses along with 30-50% Alfalfa hay to keep the protein and calcium intake up. For young horses, nursing and pregnant horses and horses in work it is also a good idea to be adding a "multi-vitamin" that is balanced for your area and hay.