Sitting

Sitting is a game mechanic used by the player and some mobs under specific circumstances.

Riding
Players and humanoid mobs can enter the sitting-like posture when riding an entity.

Wolves, parrots and cats
The player can order a tamed wolf, cat, or parrot to sit by right-clicking. These mobs stay in place, and do not move or teleport to the player unless they are attacked or the animal is forced to stand.

Wolves and cats inside minecarts or boats do not actually appear sitting.

Foxes
Foxes occasionally sit down on their own accord. This happens when they cannot find an area to sleep. Sitting happens briefly, and the fox resumes standing within seconds. Foxes also do not permanently sit within vehicles.

Pandas
Pandas also sit on their own accord, sometimes to eat.