Both fiat backed assets as well as collateralized assets pegged to fiat money are called stablecoins, because they are meant to represent units of fiat money, and therefore "stable" relative to fiat. Fiat backed stablecoins fit the definition of an IOU better, since they are cryptographic promises to pay the bearer by centralized issuing entities. Whereas collateralized stablecoins are not IOUs, they are effectively loans on other cryptoassets you deposit in a smart contract (ether, in Maker/Dai's case).