Amoeba - Monster that occurs in nebulas. TH only, no EM signature. Stealth bonus according to nebula level. No internal components. Immune to HPM and Meson. Boarding is suicide. Anything eaten increases Amoeba mass. Damage reduces mass, and can spawn small amoebas at further expense of parent mass. If lured out of the nebula, missiles cause both explosion and radiation damage. Pseudopod that acts like a Tactical Tractor. Once target is immobilized, the Amoeba moves in for the kill.
If less than half the size of target, the Amoeba will act like a boarder, and burrow through the hull. Once inside it will try to eat components and crew from the inside.
If at least twice the size of target, target is enveloped. Any smaller attached Ameoba will be immediately eaten. Digestion rate is one hit per 12 columns every 5 seconds, random distribution. Damage that penetrates results in digestion of ship components and crew, with destroyed components getting eaten rather than exploding. An enveloped ship can fire any of its weapons with a 100% hit chance. Missiles and bombs will explode against the ship's hull damaging it as well as the monster. Abandoning is suicide, since the Amoeba will eat the escape pods. Self destruct causes high damage, and has a high chance of splitting the Amoeba. Other ships attacking risks hitting the enveloped ship.
Other ships are too big to swallow, but too small to enter. The Amoeba will stick to the side and try to eat it anyway.
Snowflake - Has an EM signature, but no TH. Fires a beam weapon that only kills crew. Ignores armour, but shields can block it. Every kill increases mass. Missile damage risks breaking off pieces that can then attack independently from the mother. Heaven help you if it reaches a major population.