A neighbour of ours dealt with the exact same thing with an older (21 year old) mare of theirs yesterday. She lost total control of her back end and they all tried but couldnt get her up.
The vet came out and gave her Banamine and some other anti inflammatories, they stayed with her through the night and made sure she had access to food and water as she was eating and drinking just fine. They tried 4 times to get her up through the night and she'd get to a certain point and then collapse
They thought WNV? EPM? A stroke??? But had no real idea what they were dealing with
This morning she got up on her own, gave a shake and walked over to join her friends
Stiff and not quite herself, but seemingly not too worse for wear so they are totally and completely stumped and are also prepared for the worse with her
Theyve put her into her own paddock close to the house so they can keep an eye on her, but her appetite is great and she seems almost back to normal strangely enough!
Good luck with your guy. It cannot be colic and please don't "scare" him trying to get him up. He could really hurt himself seriously if you do ... Horses wont lie there if they can help it. They are flight animals and not comfortable being in a vulnerable position. If he could get up - he would ...