Nobody has mentioned the smaller Mule 610 XC. The XC has different gearing, larger off road tires, and a smaller steering wheel. It sips gas, fits in the back of a pick-up truck, has waterproof storage, and a great bench seat (important if your dog likes to ride!). The 4 wheel drive, and diff lock are also manual engagement, which is very reliable. It is also only about 7k brand spanking new.
I love mine, it isn't fast, but it will go absolutely anywhere.