The Wikipedia page says:

The film's scenario is at times comic or serious, and one of its peculiarities is that there never is any explanation for Ratboy's origin and existence as a human-rat hybrid.

Ratboy had a troubled production[2] and was both a critical and commercial failure

     Posted By: Paul - Wed Mar 23, 2022
     Category: Anthropomorphism | Freaks, Oddities, Quirks of Nature | Movies | 1980s

I had to look, but I thought that some of those faces were well known - Louie Anderson, Gerrit Graham, Tim Thomerson. Some of those people but have done the roles as favors.
Posted by KDP on 03/23/22 at 10:06 AM
Ratboy was played by Sharon Baird, who was a Mouseketeer in the 1950s. So the rat-boy was actually a mouse-girl.
Posted by Frank P Harris on 03/23/22 at 01:10 PM
@Frank: and all the more horrific for that.
Posted by Richard Bos on 03/26/22 at 11:44 AM
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