A grass belly is a horse that is not overweight - but not in hard work and therefore has a dropped and rounded belly. (That's my definition anyway.....)
No, grass belly is normally in the evening when a horse has been out grazing all day. It's full of grass & gas. By the morning it will have gone & the animal is back to normal. A fat horse is fat morning & night.
That's what I wanted to hear! Mine is on good grass in the day and comes in at night with a very round belly but by morning it seems to go down considerably but I was starting to doubt myself.