What does "rider" mean?

Additional senses

1. 2.One who breaks or manages a horse. Shak.
2. 3.An addition or amendment to a manuscript or other document, which is attached on a separate piece of paper; in legislative practice, an additional clause annexed to a bill while in course of passage; something extra or burdensome that is imposed. After the third reading, a foolish man stood up to propose a rider. Macaulay. This [question] was a rider which Mab found difficult to answer. A. S. Hardy.
3. 4.(Math.) A problem of more than usual difficulty added to another on an examination paper.
4. 5.Etym: [D. rijder.] A Dutch gold coin having the figure of a man on horseback stamped upon it. His moldy money ! half a dozen riders. J. Fletcher.
5. 6.(Mining) Rock material in a vein of ore, dividing it.
6. 7.(Shipbuilding) An interior rib occasionally fixed in a ship's hold, reaching from the keelson to the beame of the lower deck, to strengthen her frame. Totten.
7. 8.(Naut.) The second tier of casks in a vessel's hold.
8. 9.A small forked weight which straddles the beam of a balance, along which it can be moved in the manner of the weight on a steelyard.
9. 10.A robber. [Obs. or Prov. Eng.] Drummond. Rider's bone (Med.), a bony deposit in the muscles of the upper and inner part of the thigh, due to the pressure and irritation caused by the saddle in riding.