You could make Grievers pretty easily. Adding a poison effect for their attacks would be a little more complicated, but also doable from reading the documentation and trial/error.
Adding a location, like a maze, is a bit more complicated but is also pretty easily learned by looking at other examples and playing around with the files.
https://github.com/CleverRaven/Cataclysm-DDA/wiki/Guide-to-adding-new-content-to-CDDA-for-first-time-modders is a good place to start
You can look at other monster entries here https://github.com/CleverRaven/Cataclysm-DDA/blob/c5f2b3b24a41866a71f11ee48f093834875f008c/data/json/monsters.json
And lots of explanation of what those lines mean here
Also, the folks in the forum can be pretty helpful if you have any questions.