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It's called "N-Deklination". It happens to some masculine nouns.
Some nouns are like that. They started out that way, and phrases involving them were so frequently used that they resisted grammar changes in other parts of the language.
Languages evolve over time. You might notice that the most irregular verbs are often the most frequently used of them. They've got inertia, and it helps them to resist change.
There are some nouns that change to reflect the case they're in. With most nouns in german, only the article changes with case, but there are some nouns that do, too.
der Prinz, den/dem Prinzen, die Prinzen
der Präsident, den/dem Präsidenten, die Präsidenten
der Drache, den/dem Drachen, die Drachen
der Planet, den/dem Planeten, die Planeten
There are others, too, but those are a few examples for you.