While Buterin certainly owns a significant number of ETH, there are likely other individuals holding more ETH than him. In fact, there are several anonymous addresses on the Ethereum blockchain that hold billions of dollars worth of Ethereum.
Crypto enthusiasts and analysts describe these large holders as whales. This term comes about because of their ability to impact Ethereum as a whole with certain moves, such as transferring or selling large sums of ETH. This is a term borrowed from bitcoin, with fears that if the anonymous Satoshi Nakamoto - the co-founder of bitcoin and its largest whale - sold or moved his BTC, the entire currency could face severe disruption.