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

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