There are several well-known Bitcoin forks and airdrops that you are probably aware of. The most successful being Bitcoin Cash, which is the only likely contender that may overtake Bitcoin Core. Other novelty forks, like Bitcoin Gold or Bitcoin Diamond, still hold their value on exchanges but they may be short-lived. The ability to create a fork is not too terribly difficult if one has basic programming knowledge and too much free time on their hands. The ease of creating a fork is why there are so many and why most of them get lost in the already saturated altcoin market.