Crimson

In-Game Information
Crimson is the first monster encountered in the game. Crimson is a slow moving enemy encountered in Sector 2.

Quoted Information/Backstory
The notes state that Crimson was a new creation that didn't move much, which then got aggressive so they locked it in the vents.

"The creation of a new monster was a success! It doesn't move much though. We call it Crimson because its skin color is crimson."

"Crimson got aggressive so we locked it in the vents."