As described by AndrewAva Chow, Namecoin was the first altcoin created as such. It was announced on 2011-04-18.
However, according to the question topic "Testnet version history" the testnet1Testnet1 was reset after its coins started to getbe traded for value. Testnet 2Testnet2 was kicked off on 2011-02-03, which would allow a case to be made for Testnet 1Testnet1 to have been the first altcoin.