I changed my minecraft arms to the slim arms. I was enjoying them a bit, then checking on my villagers.. the were VERY buddy looking! I summoned a few other mobs and they were ALL buggy! I closed my world then came back in.. and now there all invisible! What on Earth? What do I do?! They keep changing too! Please tell me I don't have to delete this world.. I did so many things on it and I want to continue!! They change when a new mob is spawned.

What I expected to happen was...:
Mobs to look normal

What actually happened was...:
REALLY weird mob models!

Steps to Reproduce:
1. I don't really know why this is happening.. but I changed my arms to be slim and this all started happening. How do I fix this?!
2. ...
3. ...