This is a fully grown
G-Embryo, which comes from a host that was rejected due to DNA incompatibility. The adult has an interesting characteristic as it becomes a carrier for even more young, spewing them forth from its mouth. Whether these offspring also have the ability to grow into adults is unknown.
You face this creature in the
Marshaling Yard of the "Below Freezing Point" scenario. To defeat it, you can use practically any weapon. However, you can also use the locomotive to crush it. Activate the locomotive using the control panel when the G-Type is on the tracks. It will take more than one hit to defeat it, so you will need to repeat the process of "baiting" the creature into the path of the vehicle.