The rules for the 5E druid's Wild Shape ability simply say:
you can use your action to magically assume the shape of a beast that you have seen before
There is previous question asking if you can go beyond the exact form of a beast you've seen (like, if you want to be a dog, and you've only seen corgis, are you stuck with that?) but I think the general assumption tends to go the other way: once you've seen some example of a beast, you can become that type of beast, but only a generic form of it. For example, a comment on a question here about making up combined forms says:
you can't even wild shape into a specific kind of beast. You always become a generic, average representative of that species.
However, I don't see any wording around that. The Shapechange spell says:
You transform into an average example of that creature, one without any class levels or the Spellcasting trait.
... but I don't see anything similar for Wild Shape (and for that matter, for any of the other transformation spells and abilities in the rules).
Can a druid look exactly like the watchdogs of a particular estate? And if she can, can she also choose sometimes to just be a generic dog? Somewhat related: the Monster Manual has a mastiff, but not other types of dogs — so, if you've seen pomapoo (challenge rating presumably somewhat less than zero), can you take the form of a mastiff? What about an Irish wolfhound, as CR ½ or 1 instead of the mastiff's ⅛? What about a wolf? What about a dire wolf? Where does it stop?
Or, not to get all fixated on dogs, if you've seen a garden spider, can you become the more dangerous spider listed in the rules, with its lethal-to-many-commoners 1d4 poison bite? What about a giant spider then?
Finally, if it's not in the 5E rules, where did this idea of "average representative" come from? I know 4E had this whole thing where shapeshifting and summoning worked on a sort of platonic-ideal spirit, not real animals at all, but that doesn't seem to have carried over here. Was there something in 3.5? I remember playing it that way, but can't find a specific rule there either.
That's a lot of text, I know. Summary questions:
- Can you take the exact form of a specific, individual animal you've seen?
- 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?
- If you can generalize, can you then vary from that generalization? What defines the platonic ideal of a given creature? What if there's no Monster Manual entry that comes close?
- If you can generalize, how far away can you go? Spider to Giant Spider? Tiny Lizard to Riding Lizard? Tapir to Elephant?
- Where did this idea of generic, average example come from, if not 5E rules?
... and I'd like to see rules references for any answers, please.