Antlers
Use old hay bales put end-to-end for a quick jump. Or, if you have a sturdy hedge somewhere on the property, shave it down in the middle until you have a jump the size you want. 3nodding Yeah, what she said.
And if you can find empty 50 gallon drums, they make good jumps too, laid on their round sides... Put two end to end, and just be sure to have them braced to the ground so they don't roll if the horse hits them going over. But most horses will clear them easily.
You can find pallets (those wood flats used for stacking boxes) behind shopping centers, and most are happy for you to haul them away. You can stack two pallets on end (they have a long side and a shorter side, so do the shorter side first) and fasten them together with some cheap boards (or use one pallet to take apart for the boards) and brace them on the ground. These will be solid jumps, and won't come down easily. But they look solid to the horse, and they usually will try hard to clear them. They'd be about 3 feet tall, I think... I'll go measure mine when it's daylight and edit this with the actual dimensions.
Old tires can be strung on one heavy board or a 4x4 and used as a jump at any height, depending on how you set up the ends/standards. You can rest the rail on a couple hay bales. I would stake them into the bales to keep them steady.
If you can get enough pallets, you can take them apart for the wood and make a chicken coop, or any kind of wood jump. Or, heck, you can slant 2 pallets against 2 more pallets to make a chicken coop. (It's a triangle.)
You can paint any of these, wood, barrels or even the tires.
Fallen branches can be sort of woven together to make a jump.
OK, I'm done.
for now