It started with just a Snorlax for a base, but one day at a time, one Pokemon at a time, people suggested new additions to fuse a new gen 1 Pokemon into it and the top comment was chosen each time...
https://imgur.com/a/L6uDL62
...leading to the final monstrosity we see before us now.
https://www.reddit.com/r/pokemon/comments/1am4cv0/day_152_of_fusing_every_gen_1_pokemon_into_one_by/
https://i.imgur.com/LLwyyp6.jpeg
Professor Oak for scale:
https://i.redd.it/y5q2i3si5fhc1.png
https://i.imgur.com/PmF1p9q.jpg