It is called adipose tissue or adipose fat. It develops from chronic obesity and is a symptom of Insulin Resistance.
It, indeed, is associated with founder or a increased danger of severe founder.
Once it is there, it usually stays and the risk factor stays. I would sure keep the donkey off of good grass and would not let it even get a sniff of grain. You might visit www.safegrass.org
and see what the sugar content of different grasses is and its connection to founder on horses, ponies and donkeys.