Popper

Poppers are bioengineered terror weapons utilized by extradimensional aliens in X-Com: Apocalypse.

Overview
Poppers are about the size of an enormous bulldog. It has greenish blue, hairless skin that is covered by thick veins. It has two hindquarters, but no legs on it's front. Head has a toothy maw and two eyeballs that protrude from it's flesh and covered with only a thin layer of skin.

Although popper has mouth and teeth and a functional digestive tract, it has no use for them. All cavities inside of it's body are filled with chemical reagents, which suggests that this creature has been heavily modified from whatever it's original form may have been.

These creatures can run extremely fast, and only a trained sprint runner may have some hope of outrunning poppers.

Role in invasion
Poppers are nothing more than explosive weapons. Their sole purpose is to run and detonate themselves. Considering aliens have far more sophisticated explosives available to them, poppers are most likely terror weapons and a direct response to armed resistance by X-Com and various organisations in Mega-Primus.

Presence of a popper often indicates a presence of more intelligent alien beings such as Anthropods, Skeletoids or even a Psimorph, as they are not very clever in picking a target without guidance. Now and then if a popper is encountered in infested warehouses that have only Spitters as supervisors, they just run towards nearest human they happen to notice, much like how Brainsuckers choose their prey.

When popper has acquired a target, it somehow combines separate reagents that are stored inside of it's stomach. After a delay this mixture results in spectacular explosion. It's easy to distinguish when this reaction has been set in motion, as every tube and orifice in a popper's frame starts to generate thick smoke.