Raptors offer quality over quantity compared to swarmlings. Their leaping ability makes them much effective in battle, quickly closing the distance on enemies and surrounding them much more efficiently, as well as letting them leap up and down cliffs for superior manoeuvrability. In tandem with the Raptor's +2 damage upgrade due to their Aggressive passive ability, they are much more powerful than zerglings or swarmlings.
If the player finds themselves being attacked without defense frequently, the swarmling is a more effective response. Swarmlings are also more effective if the player prominently uses banelings, since swarmlings mutate faster and in greater numbers, in turn allowing for more banelings quicker. Otherwise, the raptor is universally superior.
During Kerrigan's resurgence, Abathur discovered the raptor strain in feral karak native to the planet Calus. The karak essence were assimilated to produce the raptor. This enhanced the quadriceps of the zergling and mutated its wing function, allowing the raptor to traverse cliffs and leap at targets from a distance. However, as a side effect the raptors were also affected with a minor increase in aggression.
The raptor used the model of the swarmling in development.
The raptor originally had an ability named "barbed talons" that dealt 10 damage to enemies upon leaping towards them. An upgrade for them named "hip extensors" extended their leaping range by 50%. A third upgrade gave them +10 life; an identical upgrade for the zergling that also affects the raptor is used in the final product.