If you watch jumpers and eventers, you will see that many of them appear to have a leather "pad" on their stomachs.
This is a "belly guard" and it's either a special girth or an attachment. The point is that jumping horses often have studs on their shoes for extra grip on landing. Many horses, when jumping high, will tuck their hooves in pretty tight against their stomachs. The guard stops the studs on their shoes from digging in.
(You might also hear of a belly guard for turnout - this is a fly sheet that covers only the horse's stomach to protect them from certain kinds of flies).
The dark brown belly guard is particularly visible on this white horse. Image source Nordelch via Wikimedia commons.