Nightmare
Nightmare is actually a very vulnerable creature, which is why it has to stay firmly locked inside its suit. There's only one Nightmare because all of the others died at some point during development, and the surviving one has a melting face in Fusion because it's body is barely hanging together. Even the X infecting it struggle to maintain structure. If exposed to the environment, it is infected by environmental microbes pretty quickly, and can't effectively fight them off.
Nightmare is pretty intelligent, comparable to an ape or dolphin. It is paranoid by design, however, and must be contained carefully because it perceives almost anything that moves as a threat. It totally destroyed several of the BSL station's BOX robots after it broke out, not even leaving anything for the X to infect. The damage it took from the BOXes created an opening in its suit which allowed the X to infect it instead. BOX number 5 lived a little longer because it went after Samus Aran instead.
