It requires a base coin in order to work. In this case the base coin is essentially a Bitcoin clone, so it is not a substitute for the Bitcoin protocol.
I think you mean distribution? After distribution all coins could be zerocoins.
Zcash is the new name for Zerocash (with some slight tweaks from the original Zerocash white paper but still overall the same design principles).
Zerocash's white paper had two types of coins in the system the regular non-anonymous basecoins and these can be minted into anonymous zerocoins.
Zerocoin (with capital Z) was an earlier design and white paper which was inferior to Zerocash. Not to be confused with the aforementioned zerocoins.