The key word here is indeed beast.
Beast is a defined type of creature within the game, as per page 6 of the Monster Manual;
Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology. Some of them have magical powers, but most are unintelligent and lack any society or language. Beasts include all varieties of ordinary animals, dinosaurs, and giant versions of animals.
Since Wild Shape specifically says
assume the shape of a beast
you are limited to only creatures that have the beast type.
Note that at level 10 a druid of the Circle of the Moon gains the ability to Wild Shape into an air, earth, fire, or water elemental, but it specifically limits it to those 4. This does not allow the druid to Wild Shape into any creature with the Elemental type (such as a Galeb Duhr or Gargoyle).
Yes, but the game doesn't tell you which. As long as your Druid isn't a newborn baby, they've likely seen some beasts in their life (especially during and after their Druid training!), and therefore can Wild Shape into those beasts.
Your background is probably the best guide to what creatures you've likely seen, as your background will imply a region that's familiar to you. Any beast common to the region in which you grew up and trained is a beast you've likely seen many times, and uncommon ones are likely to have been seen once or twice too.
This is all between you and your DM, of course, because the game doesn't rigidly define a list or rigidly define a way to create one. You're two reasonable people who enjoy playing games together though, so I'm sure you'll figure out an enjoyable, reasonable set of beasts that you can agree makes sense for a nature-loving outdoorsy person to have ever seen in their life.
Best Answer
No, Winter Wolf is a monstrosity, druids can only Wild Shape into beasts