Players can control the direction of pig or horse heads.
Players and the pig head move up and down synchronization.
This makes the pig head becomes strange.
Video:
16w06a: https://www.youtube.com/watch?v=KO8dB_uLTE8
1.8.9: https://www.youtube.com/watch?v=fII1fKlsP2U
Code analysis by @unknown can be found in this comment.
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Comments 17
This makes the pig head becomes strange.

Maybe caused by MC-96821

Confirmed for
16w06a
16w07a

Please link to this comment in the description
The following is based on decompiled version of Minecraft 1.9 using MCP 9.24 beta. All method and class names are the names used in the decompiled version.
The reason why this happens is because the method net.minecraft.entity.passive.EntityPig.moveEntityWithHeading(float, float)
is setting the pitch rotation of the pig to the pitch rotation of the player. This makes little sense because the player is only moving his head but not the carrot on a stick.
Can confirm in 21w07a.
Can confirm in 21w13a.
Can confirm in 21w16a.
Can confirm in 1.17.1. I've attached some screenshots and videos which demonstrate this issue.
[media][media][media]Can confirm in 1.18.1.
Possibly work as intended?