Learn Hardcore Korean: gangajido mokmallassneunji mureul banggeum da masyeosseo. - The puppy must have been thirsty too; it just finished all the water.

-도 means "also/too," and sometimes "even." Using 강아지도 implies that, in addition to someone/something already in mind, the puppy is included as well. It replaces any case particle that might otherwise be there.

Examples:
- 저도 가요. = I’m going, too.
- 물도 있어요? = Do you also have water?
For a stronger "even" nuance, you can use -까지 or -조차 (e.g., 강아지까지/강아지조차 ...), but -도 is the most neutral.

Why use 목말랐는지 here? What does -는지 contribute?

-는지 can mark an indirect question ("whether/if") or express uncertainty/conjecture. In result clauses like this, it often means "maybe because" / "as if" / "I guess."

So 목말랐는지 suggests a guess about the reason: “The puppy, too—maybe because it was thirsty—just drank all the water.” It’s softer than stating the cause as a fact.

Could I say 목말라서 instead of 목말랐는지? What’s the difference?

Yes:

목말라서 = a definite cause you’re asserting: “because (it) was thirsty.”
목말랐는지 = a conjectured cause: “maybe because it was thirsty.” Choose 목말라서 if you’re sure; choose 목말랐는지 if you’re inferring from what you saw.

What’s the difference between 목말랐는지 and 목마른지?

목말랐는지 uses past (았/었) and suits a situation where the result already happened; you’re inferring about a prior state.
목마른지 (present) is used for “whether it’s thirsty (now)” or for general uncertainty without a past result.
In this sentence (it already drank all the water), 목말랐는지 is the natural fit.