Jalopnik has a great discussion post up about engines, particularly car engines, with an odd number of cylinders. There are some real gems in the comments, such as this Lanz Bulldog tractor. It's a 1-cylinder engine, which is not that uncommon. Many old motorcycle designs, such as those of Royal Enfield, have only one cylinder.
But the Lanz Bulldog's two-stroke thumper engine has a displacement of 10.3 liters! Look at it shake in the above video. Also note that the steering wheel is used as a starter crank. This simple, rugged tractor design dominated the German market before World War II. John Deere bought the company in 1953.
Bonus Item: A five-cylinder rotary car engine that spins on a vertical axis.
This is the only 1906 Adams-Farwell in existence. It has an air-cooled 4-stroke rotary engine, so the crankshaft remains stable while the entire crankcase spins.