Besides the rules considerations offered by the other answers (a crucial aspect, of course), I think it is important to consider your world and story as well. Unless you're playing the most generic D&D in which rules of thumb do work, your campaign (story, setting, world) will have its own peculiarities that strongly influence the availability/accessibility of information about monsters.
For example, in a land where necromancy and the undead are practically unknown, because all the practicioners of the dark arts were eradicated a thousand years ago by an order of devout angel-worshippers, the DC to identify/learn the traits of a simple skeleton would probably way higher than what a general rule in a generic setting would require. There hasn't been anyone to pass even the most basic information on. There are no books, no records, no experienced undead-hunters around. So, when they first turn up in a remote forest, where the party is guarding a lone noble out hunting game, their chance of knowing anything about skeletons is minimal. DC 30-40, I'd say.
On the other hand, even though the generic DC for a bullette (CR7) would be around 17, the same guards could gain info by rolling for a DC of 5 (yes, five) in this example setting - because just last month a scholar (a powerful wizard, a brother of the noble) brought a number of live, shackled and dominated bullettes into the noble's court, where he conducted experiments on them, and has shared every bit of his findings with the guard, expecting an enemy warlord to try and use bullettes to attack the noble's domain... So the only question is how much attention each and every PC paid to the presentations of the wizard.
So, I'd recommend using a generic rule only when you don't have anything peculiar in mind for a given monster in your setting: they're just fillers, or their ecology very closely matches that of the generic D&D setting. However, if you have a particular story for a given species with which your PCs may have come into contact, let the story and the world – and, of course, the PCs' background(s) – dictate the DC, and allow the DC to be wildly different than what the generic rule of the RAW would suggest/set.
(Note that the same works well, imo, in the case of practically any knowledge skill.)