Certain cryptocurrencies are pegged to an underlying asset, such as the USD Coin (USDC), which is a stablecoin by Coinbase. A USDC token is issued for every $1 kept in Coinbases' reserve.
A pegged cryptocurrency can be pegged to any asset and then issued on a blockchain. As long as the cryptocurrency can be redeemed for the underlying asset, it can be considered as a pegged cryptocurrency.
A Ponzi scheme is also referred to as pyramid scheme, and typically takes the form of an investment scheme which pays existing investors with funds collected from new investors.