Okay, I found a design off a pamplet I have. Its the northwest miniaturehorse club obstacle handbook. Wish I had a scanner.
But here's what they say for instructions: 1- 4'x8' flat base reinforced plywood (looks like it has a frame with a plywood on top) and 1- small 4.5 round log to make bridge rock. Lay bridge over log at the center point.
I design the courses and want to make one. I think that if you design the bridge with a V indent in the side panels and then place it on a V (upside down) log, than it would rock. Haven't done it yet though.
Take a 1/4 sheet of plywood...thick enough to hold the horse so at least 1/2" or more....and get a 24" section of thin fence post or a 4" black plastic pipe. Drill a hole at each end of the pole/pipe and bolt it to the plywood a few inches to one end of the center. (try lying it on the pole to find out how far along) You want the plywood to sit with one end down and the other in the air and if you put the pole too close to the middle, it will not sit solidly for the horse to get on it.
Use a thick bolt to attach the pole so everything doesn't come apart if the horse is silly on it...a few screws or nails will come apart easily and be dangerous.
The one I made is 2.5' wide by 8' long. When I painted it I added some sand to give it texture and not be so slippery. I have the plywood attached to 2 2"x4" boards and then that all is laying over a landscape timber to give it the rocking ability.