Summary:
Four-legged mobs do not sit correctly in a boat with a chest. In a boat where there are 2 mobs, the four-legged turn 90 degrees so as not to get their feet into each other. It would be expected that in a boat with a chest, four-legged mobs would also be rotated, but no. The chest is under the feet of the mobs, entering the textures of the mobs.

Steps to reproduce:

1. Place a regular boat and a boat with a chest.
2. Place any mob in a regular boat.
3. Place a four-legged mob in a boat with a chest and in a regular boat.
4. Compare the way mobs are placed in a boat with a chest and in a regular boat.

Observed results:
Mobs in a boat with a chest are placed in the entire boat and enter the chest with their feet.

Expected results:
Four-legged mobs will be placed sideways in a boat with a chest.

Image: