Attach Models To Models?

[Zoldam]

Some models have other models on them, most of the time it's a parasite mob infesting the host mob (Like the vines in WoD, tentacles in some Cata mobs, etc.) how is this achieved? I know it's used as far back as Wrath since Professor Putricide is the earliest I can think of that uses this (Transition to phase 2 he drinks a potion that gives him tentacle arms), what causes this? Is it some sort of spell, or some unrelated script or something?

[Terrorblade]

Most of the time its achieved through blizzard's vehicle system.