Monster Musume / Daily Life with Monster Girl - Happy family with Cerea

Well, they could have their horse body on its side, then twist their back so that they could take the photo.

Would it be uncomfortable? Yeah. But look at the photo, it's too cute, it had to be made!

After the full moon scene, we see cerea sleeping with her entire spine (human to horse) horizontal.

I'm fairly certain she can lie down.