When a immovable entitiy(all non-mob entities, mobs with NoAI tag, slime, magma cubes, all flying mobs) "ride" a mob, this mob don't wander when it's spare. And all mobs with passengers can't be pushed by another mob with passengers.
It don't happen when a marker entity "ride" a mob.
MC-168912
Mobs with immovable passengers don't wander when they have nothing to do
Open
7
(Unassigned)
Confirmed
Mob behaviour
1.15.1, 1.15.2 Pre-Release 1, 1.15.2 Pre-release 2, 1.15.2, 20w06a, ..., 1.20.1, 1.20.4, 24w05b, 1.20.5 Release Candidate 2, 1.21
Retrieved
Interestingly mobs can go into panic mode when hit, but will stop moving after