I have an expansion pack* that lets you play as the Undermother with an Invisible Swamp Donkey as a companion. You can buy it from the traveling hacker found randomly in the Sewer of Horrors.
The invisible swamp donkey isn't always invisible, temporary invisibility is just a skill of his (5, 10, 15 seconds for Level 1,2,3). His go-to combat skill is Donkey Kick which does a lot of damage with a serious knock back.
*totally not true. I can barely code a combination lock let alone a game re-write. But hey, this is how rumors get started