Lot's of altcoins have been forked from btc, then got changed, then other people forked those coins again.
It goes on and on, a fork isn't anything special.
To clarify for OP: In the context of Bitcoin a fork can mean 2 things:
1) A code fork, meaning a new alt coin that is based on Bitcoin's code but whose transaction history has nothing to do with Bitcoin. This is the more than 1000 alt coins the previous replies refer to.
2) A blockchain hardfork, meaning a new alt coin -- or arguably, Bitcoin upgrade -- that not only shares parts of Bitcoin's code, but also its transaction history up until the point of the fork.
BCH is the first alt coin that forked the Bitcoin blockchain, ie. it shares the transaction history of Bitcoin right until the time at which the hardfork occured. Bitcoin Gold and B2X will be other blockchain hardforks, meaning if you own BTC at the time of the fork (and hold the private keys) you'll own coins on both sides of the fork once the transaction histories start to diverge.