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
A druid can only assume the shape of a beast. Any combination of two animals would be a monstrosity.
Beasts include “all variety of ordinary animals” (MM p. 6) while “Monstrosities are monsters in the strictest sense—frightening creatures that are not ordinary, not truly natural.” (MM p. 6)
Even more to the point, later on the same page:
A magical experiment to combine creatures results in a monstrosity not a beast — and thus would be an invalid form for a druid to assume.
D&D rules follow a classical definition of “monster”
Back in Classical Lit they taught me that the Greeks considered things “monsters” that did not fit into proper categories, that is, they were “neither fish nor fowl.”
Many of these classical monsters are represented in the Monster Manual and they, indeed, are all classified as Monstrosity: centaurs, chimeras, cockatrice, griffons, harpies, hippogriffs, manticores, medusas, and sphinxes.
Not all hybrids are monstrous, magical hybrids
Natural hybrids (such as mules or hinnies) are a different case entirely. A druid would simply need to have seen such a hybrid to assume the form.