When you try to summon any entity riding another entity, it only summons the "main" entity.
For example:
/summon Zombie ~ ~ ~ {Riding:{id:Zombie}} -- Summons only one zombie.
MC-99585
Can't summon a riding entities
Resolved
Duplicate
0
Duplicate of MC-97392. The riding tag was replaced with passengers tag in 1.9. See that report for details.