The Little Prince meets some animals that can talk: for example, a fox and a snake.
Well, we know that they can't actually talk, but do they really exist or are they just figments of the Little Prince's imagination?
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Of course this is a fictional book, but in-universe they are real.
They teach him (true) facts about the world. The fact that they teach him is pretty obvious, but think about it: how would he learn new information from himself? If they were figments of his imagination, they wouldn't know any more than him.
For a slightly better argument, note that the pilot sees the snake at one point:
There before me, facing the little prince, was one of those yellow snakes that take just thirty seconds to bring your life to an end.
Of course, he didn't see (or hear) it talking, but it was at least there.