The sweel is an aquatic creature that was never implemented into the Thief universe. In early concept sketches, it has the appearance of a very long-bodied rodent with large whiskers and tiny limbs. The name is derived from the root eel, while "sw" may be an abbreviation for sewer.
The design was apparently rejected very early in development and has no known mesh data or sound files. Some attempts have been made to create a usable sweel for fan missions, but with limited effectiveness.