Yes, it makes sense. People also commonly say, "This species is endangered," or "It's an endangered species."

Predicative vs. Attributive: As it is used, it is predicative. This is because the noun is followed by the verb "be/are", and then comes the adjective. An attributive adjective comes immediately before the noun. For example: "These endangered animals [...]." Note that when using predicative adjectives, you can form a complete sentence. When you use an attributive adjective, it is only part of a larger sentence.

Yes. You can also say "This is an endangered animal" Endangered is a status, almost like a title given to a group of animals , it is also something that happens to animals (they become endangered) if they are at risk of being killed off.

Yes, "These animals are endangered" sounds completely correct/normal to me. I have heard this sentence many times.