Horses don't lie down much. They can lock their legs and sleep standing up, but they still need to get off their feet for a space of time. And why not lie down in the sun on the warm ground?
They might only spend an hour out of 24 like this, and even less laid right out with their heads down. It kind of freaks me out when they flatten out. They look dead. I'm looking for the rise of the ribcage and the twitch of an ear to show me they're just sleeping.
I am always struck by how different they look like this. Phoenix reminds me of a beached sea mammal. Copper looks like a kitten with her legs all tucked under her chest.
This is an extremely vulnerable position for a horse to be in. Instinct tells them not to be this way, just in case a mountain lion comes along looking for a nice big hooved snack. A lot of horses won't let you get this close to them when they're lying down. These two are ridiculously tame, but Bucky still thought it was really cool to get this close to them before they decided they ought to get up and shake it off.