This format restriction can be applied to any of the main-stream formats; Standard, Extended, Legacy, or Vintage. Standard and Extended are very well represented online, since the digital and paper card pools are identical, but the eternal formats tend to differ. Online the Legacy, and Vitage formats are combined in Classic, while in Paper it is simply called Pauper: Eternal. The key difference in Classic and P:E is that not all cards have been printed into the digital card pool, so the paper options remain much deeper, and in my mind the most interesting of the Pauper formats.
Despite being an eternal format, the barriers to entry are extremely low. Of the roughly 4,000 cards printed at common, less then fifty cost a dollar or more. Many of those are due to collectible issues, and not their play value in the format. It's very easy to build a deck for under $5, and you have to work at it to go over $20.
Zoo is a remarkably easy deck to build, but notoriously difficult to perfect, and I think this Pauper version will be no exception. I'm able to simply port over a great deal of my current Legacy list, since it his common heavy to begin with. The land base has been updated to 6 each Forest, Mountain, and Plains with 4 Terramorphic Expanses, to round it out.
Kird Ape x4
Qasali Pridemage x4
Wild Nacatl x4
Lightning Bolt x4
Terramorphic Expanse x4
Forest, Mountain, Plains 6 each.
This leaves a lot of open slots to fill, and Invigorate really looks like it doesn't belong any more. By my count, there will be 22 cards to be addressed.