Tractor

The Tractor is a tractor. You can tow cars with it like the Tow Truck.

More info:
gta.wikia.com/wiki/Tractor
https://www.grandtheftwiki.com/Tractor

Mower

The Mower is a lawnmower.

One can be found by a mansion on the Vinewood hills.

More info:
gta.wikia.com/wiki/Mower
https://www.grandtheftwiki.com/Mower

Combine Harvester

The Combine Harvester is a piece of farm equipment. It's width and rear-wheel steer make it the most difficult vehicle in the game for me to drive.

You can guess what happens if you run over pedestrians with it.

You can find them spawned at Blueberry Acres, the Farm, and Helen Wankstein's farm. They are often locked, so you usually need it to be daytime so they are occupied and then shoot the driver.

More info:
gta.wikia.com/wiki/Combine_Harvester
https://www.grandtheftwiki.com/Combine_Harvester