Learn German: Die Erdlinge sind nicht nur mutig, sondern auch schlau. - The earthlings are not only brave, but also smart.

Yes! Nouns that end in the suffix -ling are wonderfully predictable. They are always masculine in the singular (like der Erdling, der Schmetterling, der Feigling) and their plural is always formed by simply adding -e without adding any umlauts.

Why is sondern used here for "but" instead of aber?

The phrase nicht nur ..., sondern auch ... is a fixed correlative conjunction meaning "not only ..., but also ...". More broadly, sondern is used instead of aber whenever the first part of a sentence contains a negation (like nicht or kein) and the second part offers a direct correction or alternative.

Since die Erdlinge is plural, shouldn't mutig and schlau have plural endings?

No, because they are predicate adjectives. This means they follow a linking verb (sind) to describe the subject, rather than sitting directly in front of a noun. In German, adjectives only take endings when they are placed immediately before the noun they modify.

Is Erdling formed the exact same way as the English word "earthling"?