The Pokemon in the Hidden Grottoes are specific to their location. In Route 2, the Pokemon in there will always be either a Nidoran (female), a Nidoran (male), a Granbull, or a Watchog. You will never find a [for example...] Manectric in the Route 2 Hidden Grotto. The natures are not set. If you capture/defeat the Pokemon in the Hidden Grotto, 265 steps later, there is a 5% chance of the Hidden Grotto regenerating and there being new contents in the Hidden Grotto.
EDIT: No, you can't save in front of a Hidden Grotto to see if anything changes. Once a Hidden Grotto regenerates, everything is set. At least, until you defeat/capture the Pokemon or pick up the item in the Hidden Grotto, and walk 256 steps to have a 5% chance of regenerating the empty Hidden Grotto.
So if you see an item, or a Pokemon other than what you're looking for, quickly defeat it or obtain the item so you can get more chances to find the Pokemon you're looking for!