From Dr. Kellon:  "Myth: Beet pulp is high in insoluble fiber and poorly digestible. Exactly the opposite is true. It is lower in insoluble fiber than grass/hay, high in soluble fiber and very easily digested in the large intestine by fermentation. Because of this high soluble fiber content, beet pulp is also an excellent prebiotic."

But does it cause gut irritation in some horses?  I have twice had horses go off their feed and have increased intestinal sounds when beet pulp was introduced (possibly too quickly).  One horse, who had a history of what appeared to be gas colic, and appeared to be gassy in this circumstance, colicked and died overnight shortly thereafter after what I now realized was days of intermittent gassiness and mild discomfort.   I have no idea what was going on with him, but he was the type of horse who had colicked a couple of times over the years, though resolved with treatment, so he was susceptible.  I always wondered if it was the beet pulp, and I am responsible.  My vet is not very good at sleuthing out the causes for things, so she was no help in figuring out the roots of the gassiness, and I had never heard of Gas-X.  

So it was unlikely to have been beet pulp related?