@thejarren - Immobile Mobs

(Immobile but can be pushed)
/summon Creeper ~ ~1 ~ {Attributes:[{Name:generic.movementSpeed,Base:0}]}

OR

(Immobile, can't be pushed, but uses visible item)
/summon Creeper ~ ~1 ~ {Riding:{id:Item,Item:{id:minecraft:snow_layer,Count:1},Age:-32768,PickupDelay:32767}}

OR

(Immobile, can't be pushed so long as the squid remains underground)
/summon Creeper ~ ~ ~ {Riding:{id:Squid,Invulnerable:1,ActiveEffects:[{Id:14,Duration:2147483647,ShowParticles:0b}]}}