|Slithering Tracker by Bill Willingham|
Here's yet another piece of original artwork found along with the Masher, Drow Captain and Bugbear Chieftain. This one illustrates the Slithering Tracker from the Monster Manual, which is described as "transparent and almost impossible to discover" and having a "semi-fluid body" that can "flow through openings as small as a rat hole or a large crack under a door". I hadn't really thought of the Tracker as one of the "clean-up crew", but this illustration makes me think it's related to the oozes, jellies, and puddings. Like the Masher, this is another monster that might have gotten more attention with an illustration. I always assumed there was no illustration because it was invisible!
I hadn't seen this piece until recently when I happened across it on a page at Tome of Treasures. According to the original post on the Acaeum the ebay lot where these were found also included a Giant Skunk by Willingham and an unattributed Shadow, each of which was intended to be added to the Monster Manual (but never were). I haven't come across scans of the Giank Skunk or Shadow anywhere.
If you'd like to see more Willingham art, there's a recent thread over on the OD&D Discussion forums dedicated to his work that has some scans and links. In the first post of the second page I started compiling a bibliography of his work.