You beat me to it npalen. IH made lots of hydrostatic tractors. I'm not positive what the largest was, but they definitely put a hydro on a 1066@125 PTO HP and an operating weight of 12,000#. That, my friends is even bigger than a utility tractor (at least by the standards of the day). A partial list of their hydro tractors includes: 544H, 656H, 666h, 70, 84, 86, 186, 100, 966h, 1066h, 826, and 1026. The small frame ones are still used extensively for veggie growing. They offer an infinitely adjustable ground speed relative to PTO speed and can be had cheap in many cases. Great for running tillers and transplanters.
Here's one for sale near me:
IH 186 Hydro Tractor
Regarding the 656, they aren't all hydro. Probably a small percentage, really. The standard transmission is a 5F1R with a 10F2R torque amplifier transmission also available. Interestingly, the hydro transmission on the 656 would do 21 mph on the road.