Chimera is an AI that was initially created by the Cheshires.
They would no longer cite it as a friend, though, and have tried to shut it off; (un)fortunately to no avail. It escaped and is currently being protected by Nightshade, who have a bit of a love-hate relationship with it and are trying to ensure it can function again and make more Thorns - something it currently cannot do because it no longer has access to the machinery it used to make them with.
Most Cheshires are not actively trying to find and shut down Chimera, but if they come across it they would try to destroy it.
Name
The name Chimera was given to the AI by lucid humans and Thorns. Cheshires that choose to speak to humans or Thorns about the AI will also refer to it as Chimera, but that's not the name they use for it amongst each other. (Sokuska is their name for it, but chances are you'll never come across that in canon.)
Creation
Chimera was designed to find and create a cure for an infection that's taken to a scant few Cheshires (Skath). Cheshire-human hybrids were one way it tried to achieve that. There are Cheshire-<other-carbon-based-lifeform> hybrids out there as well, but those have proven to be less interesting to the AI because they're not sapient and thus not as easily comparable in regards to 'how does stuff I do (including manipulating what you even are) affect you'.
In effect, it's accidentally 'good' for humans, since it wasn't programmed with an express “make sure you don't accidentally give the humans anything that could be a problem for us” terminal value. It's certainly ultimately responsible for Nightshade.