Yamaha entered a 2x2 motorcycle in the 2004 Paris Dakar off road race, utilizing a hydraulic drive to the front wheel.
This is not a classical Hybrid vehicle application (i.e. neither parallel nor serial) as the rear wheel is traditionally chain driven.
Yamaha has however shown the world that they are capable to produce durable and lightweight hydraulic drives.
Expect to see a lot more on this front in the coming years.
The 2x2 Yamaha with hydraulic front wheel drive.
Here is a desctiption from Yamaha on how the 2-trac hydraulic drive system works:
Yamaha patented system uses a hydraulic pump located on top of the gearbox and driven by a chain in oil bath (see red circle) which actuates hydraulic fluid via high pressure hoses to the front wheel. The special hub in the front wheel is a "hydraulic motor" (see blue circle), which is set in motion through the pressure of the hydraulics.
This construction allows a "smart operation": the pressure and consequently the pulling power on the front wheel varies the more the rear wheel slips. The system can regulate itself and applies always the best ratio of power between front and rear. You can also adjust the system, depending on usage of the machine and apply more basic thrust in front or less.