Why do they call certain Toyota Tacoma models, a 'PreRunner'?

The name Pre-Runner comes from off-road racing events like the Baja 1000. Competitors will take a vehicle called a pre-runner down the racecourse prior to the race to familiarize themselves with the layout. The Pre-Runners are production based trucks that have to be tough enough to survive the off-road racecourse, but they are less expensive than the full-on race trucks, since they won't be running wide open, taking jumps, or things of that nature.

Toyota adopted the name for the truck because it was less expensive than a 4x4 Tacoma, but nicer, and with more heavy duty off-road parts than the non-PreRunner Tacos.

PreRunners are 2 wheel drive (rwd). A PreRunner is basically the 4 wheel-drive model without the front axle and transfer case. For example, when I buy suspension parts for a PreRunner, I order from the 4wd parts list.