Bit burrs are discs with spikes on one side used to prevent a pony from leaning to the side. The spikes on a bit burr differentiate it from a bit guard. Most often, you will find them made of rubber (with rubber spikes), but leather ones (with stiff bristles) are not uncommon, and both can be purchased from bio-equine tack shops. The rubber ones are well suited to ponyplay, and they are quite cheap (a few dollars). As with bits, bit burrs are commonly found in bio-equine tack shops and are easily adapted to ponyplay.
The bit burrs attach to each side of the pony's bit such that the side with the rubber spikes is facing and lightly touching the pony's cheeks. When rein action is applied to one side when steering the pony, the burrs exert pressure on the outside of the cheeks. This makes it much more uncomfortable for the pony to resist the action of the reins. Bit burrs are useful for ponies that have a tendency to resist the action of the reins.