The Rider NPC is a robot, dressed in the traditional garb of participants in a British fox hunt, mounted on a horse, which has jet engines in place of actual legs. These NPCs damage the player by riding over them, burning them with the blast from their mount's engines. When one spots you, it makes a hunting horn noise (which sounds like a locomotive horn) then charges, it is very difficult (if not impossible) to dodge.
An effective way to survive being spotted by a Rider is to reach a village. By hiding behind houses, it is possible to use them as shields against the Rider's charge long enough for it to lose sight of the player. At this point, surviving the Rider is only a matter of hiding, which shouldn't be too hard in a village if the robot is alone.
One close-ranged shotgun blast, a rifle shot, a blunderbuss shot, a full round of revolver bullets or four hits with the axe is enough to take them out. Riders can drop scanners, a map, sticks of dynamite, various alcoholic beverages, dead animals, and even rifle ammo. It should be noted, however, that the rider's horse can withstand at least 2 rifle bullets before being destroyed, however the above damage statistics refer to a hit on the considerably more fragile rider itself, rather than its mount.
Some mention of them here in a kickstarter update
Our most ambitious NPCs will take on the form of a classic British fox hunt, one of the favoured bloodsports of the rich and landed. These red-coated rider robots will be the most merciless - and the fastest - hunters in the archipelago. Once they have you sighted, well, run, Fox, run.