Something that has always confused me about the Knowledge skill in D&D 3.5 is that identifying a monster with advancement is more difficult only because the HD increase. The DC of the knowledge check according to the SRD is 10 + the monster's HD. By this logic two red dragons, one a wyrmling with 7 HD takes a DC 17 check to identify and the other an adult with 22 HD takes a DC 32 check to identify. It doesn't make sense that the same creature gets harder to identify when it gets older. I recall reading somewhere that you ignore advancement with monsters when determining the DC and use the base HD. If this is the case, what do you do about monsters that don't have a base HD, like a vampire for example.
My main questions here are:
Does the DC for identifying a creature use the specific creature's HD or the base HD for the type of creature. If it does use the base HD then what would the DC be for a creature that does not have a base HD.
Best Answer
Sometimes the Rules Are Guidelines...
According to the Player's Handbook, "[Y]ou can use [Knowledge skills] to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster's HD. A successful check allows you to remember a bit of useful information about that monster" (78). And there just ain't further guidelines; I mean, there's that ominously pregnant in general squeezed in there to allow DM fudging, of course, but otherwise the DM's on his own.
...And Then There's Lore
That changes in, I think (and comments can totally correct me if I'm wrong), July 2006, when Monster Manual IV included a lore entry for each creature. In the Manual's introduction under the Lore heading, it says of the Player's Handbook's use of Knowledge skills to identify creatures
Then, without further preamble or explanation, each of these lore entries instead of being based on the creature's HD is based on the creature's Challenge Rating (a system later adopted by Pathfinder), with the DC starting at 10+CR, a change which makes sense to a degree yet leaves unaddressed corner cases like advanced and templated creatures.
I did some research and there was no Design & Development column (yes, they're really called that--O, you wacky Wizards!) about the Monster Manual IV, and Dave Noonan doesn't mention the lore entries in his articles here and here about the design and development of the Monster Manual V, so I don't know the rationale behind the change from identification based on HD to identification based on CR. (By the way, these are the last 2 Design & Development articles published, which is a shame.) (As a further aside, I was disappointed at the removal of both the Environment and Treasure lines in the later creature entries, but that's another issue.)
Bold Assumptions
The gnoll entry in Monster Manual IV (67-71) presents gnolls of CRs of 3, 4, and 6 yet provides lore results based mathematically on the CR 1 gnoll. The design intent seems to be that when an advanced or classed creature is encountered the CR of the base creature is used to gather its lore. Templates, appear--based on the templates Lolth Touched (MM4 94), God-blooded (MM5 66), and Phantom (MM5 130)--to require a Knowledge skill check with either a DC of 15 + the creature's new CR or a DC of 15 straight up to identify the template. That's just for the template, though; identifying the underlying creature is a separate Knowledge skill check.
More Lore
An effort was made on the Wizards of the Coast message boards to compose lore entries for creatures that lacked them. The results are here, although a more table-friendly format's here.
"But What About Dragons?"
Drow of the Underdark gives us the deep dragon (114-7), the first dragon I could find post-Monster Manual IV that has age categories and a lore entry. The Knowledge check DC for the deep dragon is 15 + the dragon's CR, implying that dragons of different CRs are different creatures therefore requiring separate Knowledge checks.