Can a druid shapechange into the form of a specific, individual beast?
Yes, it must be of a beast and of a beast that the druid has been seen before, and within the limits of the druids level allowances to shift into. It continues on for an entire half a page with details on wild shape. I highly recommend reading it through its entirity as it will clear up alot of your assumptions. PHB 66/67
Wild Shape vs Shapeshifting. Wild Shape is not Shapeshifting, if it was it would be called shapeshifting.
Can a druid look exactly like the watchdogs of a particular estate?
According to the answer to the first question, yes if they have seen the watchdogs of the particular estate in question, or at least of a watchdog within reason for the GM to allow it, at the GMs’s discrretion.
Spider situation. If the druid has seen a Giant Spider but not a Giant Wolf Spider, then the druid would be able to Wild Shape into the Giant Spider and not the Giant Wolf Spider as the druid has not observed a Giant Wolf Spider. Or if the druid has seen both the Giant Spider and the Giant Wolf Spider but NOT the Phase Spider, then the druid would not be able to Wild Shape into a Phase Spider but would be able to into a Giant Spider or Giant Wolf Spider. According to PHB 66/67 it entirely depends on what the druid has witnessed.
Can you take the exact form of a specific, individual animal you’ve seen? Yes
Can you take the form of a “generic” animal of the same general species or name or categorization as a specific individual you’ve seen? No, ie Giant Spider vs Giant Wolf Spider vs Phase Spider.
If you can generalize, can you then vary from that generalization? GM’s discretion at this point. Work with the GM on what would be too far and what would be allowed.
What defines the platonic ideal of a given creature?
What if there’s no Monster Manual entry that comes close? Check online for homebrew content in the community, there are tons out there. Check the DM’s Guild for options there too.
If you can generalize, how far away can you go? Per the GMs discretion.
Spider to Giant Spider? No, beast type change would not allow that.
Tiny Lizard to Riding Lizard? Nope, type change would not allow that.
Tapir to Elephant? Way too different to allow that.
Rhino to Elephant? No. Hippo to rhino? No.
It if is a Different beast than what the druid has witnessed it cant be allowed per the rules on PHB 66/67, as it MUST be of a beast the druid has seen.
Where did this idea of generic, average example come from, if not 5E rules? This one not sure as the community did a lot of homebrew after the SRD 3.5 and SRD 5e were released. Though it may be from before that.