Used Dirt Bikes
They are very tall for ground clearance, light to make manouevering and picking up easy, have long suspension travel again for ground clearance and the ability to go over rough tracks without "bottoming out", have small but very tourqey single cylinder
engines which forgo top speed for low down grunt (useful for hill climbs etc), they have a simple rugged construction with minimal body work to minimize damage and have large wheels and knobly tires - both of which assist the rider in rough terrain.
There are also different types off off-road motorcycles or dirt bikes refined further for their particular purpose.
For example, Motocross bikes used for racing on man-made tracks with big jumps have even longer travel suspension to absorb the impacts and small fuel capacities while Enduro bikes are very similar to Motocross bikes but are used primarily for trail riding and racing through forests and as such have a larger fuel capacity.
Many of these very niche specific models will not be road legal while many dirt bikes are and do handle admirably both on the road and in the dirt. They are therefore in many ways the forebearers of the large dual-sport adventure motorcycles which have become increasingly popular in recent years.