You just need to see one beast of its kind, not every possible phenotypic category of a kind of beast. If you did, it would specify that with something like
... the shape of a beast of a [gender/age/size/colour/seasonal pelt] that you have seen before.
It only specifies that you have seen that kind of beast before, nothing more.
Yes it does
Yes, you can add beast forms to your Wild Shape repertoire of beasts seen by casting conjure animals. The spell explicitly says the conjured creatures literally are beasts, and a creature of the type beast is all Wild Shape needs you to see in order to add its form to your repertoire.
Though someone might by tempted to quibble that they're not real beasts because they're really fey spirits clothed in the shape of beasts, look at it this way: First the Druid conjures a bunch of spirits that take on the form and behaviour of the real beasts they look like. The creatures so conjured proceed to behave and look exactly like the real thing. What better opportunity is there for a shapechanger to learn a new form to wear, than to see a nature spirit do the exact same thing as they're wanting to learn to do?
But it's of limited use
What you might actually run afoul of is limitations on metagaming, or a DM whose setting doesn't include just any creature you can think of (even if it's in the Monster Manual).
- A DM who wants to limit metagaming would be within their rights to ask you to justify your choice. If you try to summon a Giant Owl but your druid has never seen one before, a DM could easily say "How do you know those exist? You've never seen one. How is your druid "choosing" an animal (s)he has never seen or heard of before?"
- A DM who has developed a custom setting isn't straightjacketed by the Monster Manual and doesn't have to include everything in it. Giant Owls might not exist in a DM's campaign.
This also means that this is much less of a loophole than it might seem — after all, how big is the difference between the set of creatures a druid has personally seen and the set of creatures the druid knows about well enough to deliberately try to conjure? I can't imagine it's a very large number.
But in principle, yes, a conjured animal inhabited by a fey spirit is plenty to learn that form from. Just mind that, if you can conjure a particular animal, it's highly likely your druid has already seen it anyway. This makes this method of acquiring new forms both limited in utility and not much of a loophole to worry about, either way.
Best Answer
Unfun answer: Strict RAW - No
Wild Shape
A strict RAW reading of this would say no for the simple fact that seeing an illusion of something is not actually seeing the creature in the plain English sense of the word.
Jeremy Crawford said something that works well as a rationalization for this requirement too:
He is actually talking about Shapechange when he says that but Shapechange has the exact same wording for this as Wild Shape so it should be applicable.
However, it is easy to see that this can easily devolve into, well what DOES qualify as seeing the creature then? Crystal ball? Telescope? What counts and what doesn't?
Better answer: DM Fiat
The RAW ruling seems overly strict for me as well as prone to misinterpretation and argument. There are just some places where the rules do not have a clear answer. This is why we have a DM to fill in the gaps.
I think having the DM rule on this on a case-by-case basis will make for a more fun, more empowering result. Especially since the Wild Shape list is so limited that it is not really prone to abuse.
And I'm not the only one who thinks that: In that same interview Crawford is asked:
To which he responds that he would often rule in cases like this that
For DMs, instead of saying no to this request, why not make it interesting? Since they are not technically meeting the requirements for the ability, have the animal look a bit weird or have abilities that are slightly off.
Depending on the nature of the illusion, how long the player saw it for, how well they saw it, etc the DM could have many creative responses to attempting this. It could lead to some fun and interesting stories, which is what this game is all about right?