Drawing for Beginners: the Triangle
Drawing for Beginners: the Rectangle was not the only film in its series. I don't know whether the Circle and Square installments are still extent, but somebody sold a copy of the Triangle last year, so it's out there, and I think it's safe to assume it is worthy of Rifftrax.
A few stills from the film are on the eBay auction, including some sort of tree dinosaur. http://www.ebay.com/itm/16-mm-Drawing-For-Beginners-The-Triangle-400-039-/361658877539?nma=true&si=FIM%252BS%252FBNp5mlh%252BgVgo4dHv%252Bgdh0%253D&origcvip=true&rt=nc&trksid=p2047675.l2557