This? This is cool. Hang on while I turn into a giant horse nerd for a minute here.
Horses will not lie down unless they feel totally at ease and unthreatened, and especially in the flat-out position. They only lie flat like the two in the front if they’re going to do some REM sleep, which only lasts about 45 minutes. Normally a horse will lie in sternal recumbency, like the two in the back.
The fact that these ponies feel comfy enough to lie down and nap on a beach full of people is impressive— it means they live a life generally free from stress and have learned that they can trust humans. I’d love to know which group of wild ponies this is!
Fun fact: in a herd of horses taking a nap, there will always be at least one who is standing, although they may be relaxed (see cocked leg on rightmost horse). This horse has the job of being the lookout.