Old tyres (tires) can be useful around the stable yard they're excellent for holding feed and water buckets making the bucket less likely to be kicked over.
They make great hay feeders in the field, especially on a windy day, saves those "hay tumbleweeds" blowing out of the paddock.
They make useful cavaletti blocks
and other jump stands, they can also be sunk into the ground on their sides for more permanent jumps.
Large tractor type tyres can be packed with earth to make a horse agility platform for your horse to stand on and if you've access to countless tyres ( and manpower) you could even build your own round pen from recycled tires like this one here