My grade pony Salt was like that. I got him when he was in his mid teens at best guess (he came through a rescue who got him at an auction, with no backstory). Any time I looked out to the manure pile, there was Salt, lying down napping. It was so often and for such long duration that a neighbor of the first place I boarded him after selling our property said he thought he was dead. I heard that a lot throughout his time with me–you had to look carefully to see his ribs moving and an occasional ear twitch. He loved his naps, and would grumble if you got him up to drive him.
Rebecca