I don't know why but I've seen a lot of these around recently - perhaps it's the time of year. This is the first I've seen inside the house, so I swiftly captured it in a pint glass to be photographed. I had to be quick because they're pretty swift across the floor! Maybe it has been eating the woodlice that also seem to have been swelling in numbers of late, though I'm not sure which I'd prefer scurrying around the living room.
It is, as the title suggests, a Devil's coach horse beetle, notable for it's very short wing cases (elytra) and long, flexible abdomen which is curls upwards when threatened. It can also emit a nast odour from the end of the abdomen and can give a painful bite. In the photo above you can just about see the very fine hairs that cover it, reflecting a shimmering green from the abdomen.
As well as my never-ending supply of woodlice, it eats worms, other invertebrates and carrion. The Rove beetle family, of which it is a part, is so called because they are always on the move. It gets its own rather dark name from various superstitions linking it to the devil. The wikipedia article has more information on the name.