Massive Pokemon GSC leak has source code and beta creature designs for second-gen games
Building upon the Space World GS demo that was leaked a while back, and a source code leak of Pokemon Blue, is a brand new discovery, involving the second-generation Pokemon games once more. Supposedly, multiple early builds of Pokemon Gold/Silver are out and about on the internet, courtesy of an anonymous 4chan user. Despite the dubious source, the alleged leak contains data for beta designs of the gen 2 Pokedex, complete with unfinished beta designs of many Pokemon featured throughout the early Gold/Silver versions, along with the source code for Pokemon Crystal. The file is 784.2 MiB.
There is a veritable treasure trove of information to dig through, including the wildly different concept sprites for some of the Johto Pokemon, a GameBoy emulator created for testing purposes by Nintendo themselves, debug builds, and much more.
The builds themselves cannot be shared here, though they are floating around on the internet.
Source: beta build screencaps