I don't think anyone has found any beta for any of the Sly games, but there is a couple of demos with beta content(E3, Jampack, others, etc.) for the first game. You can easily google it if you are interested. Who knows? Maybe one day a beta will pop up (I still have hope for this). Also, during the Mz. Ruby boss fight I think Sly says:"Dig that voodoo!" or "Zig zag voodoo" with each button representing one of those words or a syllable of the words.
Yep, I saw someone else had dumped the demos online, though they were publicly available. I do know that on NintendoAge, there is someone on there who owns a "Review build" of one of the Sly games, but it didn't mention which one. Oh, and in regards to what Sly says during the boss fight, yes, you're correct.
My holy grails uh... ~A Medievil 1 prototype with the cut dragon chase level ~The prototype of the earliest shown version of Pokemon Diamond and Pearl ~ A Oblivion proto that includes the cut city sutch
Any pre-final version of Super Mario Bros 3. The grasslands parabeetle level and different World 1 map on the back of certain versions of the US box intrigued the hell out of me when I was a kid (and they still do).
Not sure how that made it to the box, it confused the hell out of me too. Where is this level!? So much time wasted trying to access new "secret" parts of the world maps, lol.