As the title says, horses are rendered as pigs since I installed Optifine. This is on a multiplayer server and even when no shaders or texture pack activated the horses are still pigs. Other players without Optifine see the horses fine.

OptiFine_1.18.1_HD_U_H4.jar

This is most likely a problem with optifine. You can try opening an issue on it's bug tracker: https://github.com/sp614x/optifine/issues
Good luck!