Learn Dutch: De hond en de kat beschermen elkaar nooit. - The dog and the cat never protect each other.

Elkaar means "each other" or "one another". Here it acts as the direct object of the verb beschermen (to protect), showing that the action goes both ways between the dog and the cat.

Can we use elkaar for animals, or is it only for people?

You can use elkaar for people, animals, and even things! In Dutch, it simply means "each other" regardless of who or what is performing the action.

If I wanted to say "they protect themselves", would I still use elkaar?

No. If you use elkaar, it means they protect each other. If you want to say they protect themselves (the dog protects the dog, the cat protects the cat), you would use the reflexive pronoun zichzelf.

Why is nooit placed at the very end of the sentence?

In Dutch word order, adverbs of time like nooit (never) usually come after a specific direct object. Since the pronoun elkaar is the direct object here, nooit must go right after it at the end of the clause.