A trader is in the natural short position if he owns the quote currency of the trading pair. For example, if you hold USD at the exchange which offers BTC/USD trading, you are in the natural short position, and you will benefit from the price falling since this will allow you to buy more BTC at the lower prices.
Note that you don’t have to borrow the base currency (BTC in our case) to sell it short and repurchase it later at the lower price to be natural short. Anyone who holds the quote currency at the spot exchange is in the natural short position.
Speculators who lend their USD for margin trading at exchanges such as Bitfinex, for example, are in the natural short position since they hold USD (quote currency) to lend it out and get paid for it, but will not benefit from the price appreciation of the asset.