Buggy shadows

Based on the screenshots, I would say it's probably a mod or resource pack problem. Either way, the moderators are probably going to flag this as invalid because it is a modded client.

By the way, how did you get that horse upside down?

Name a mob "Grumm" or "Dinnerbone" and it will turn upside down

You were right. This is a resource pack problem. Mods, you may close this now. 🙂