The three photos above are Whipple-trees, used when horses (etc.) pull wagons.
Note: Blaine Whipple reports that the objects in the photos are really called "single trees"--using two of them makes a Whipple-tree:
Whipple-tree: a pivoted or swinging bar to which the traces, or tugs, of a harness are fastened, and by which a carriage, a plow, or the like, is drawn.
A single-tree can be a whipple-tree if the item being drawn is a one-horse implement, e.g. shay, sleigh, etc.
Submitted by George Carroll Whipple III