I've just asked one of my native-speaking friends, and he said that although "pentu" indeed should mean specifically dog puppy "by default", in the absence of context he would assume that "pentu" is "a cub", not necessary a dog. So, for dogs he would use "koiranpentu", and for kittens - "kissanpentu". There are no reasons to translate "pentu" specifically as kitten unless it is clear from the context. Just for fun, I many times saw and heard biologists to refer to the rat pup as "rottapoika" or even "rottanpoika", and the same friend said that this is not some kind of humor or professional jargon.
"Pentu" is generic to some animals, but not necessarily to all animals. It is used with cats, dogs most of the time, but like Alex said, to be clear you could use "koiranpentu" or "kissanpentu".
Most common/generic term for a young animal of a species is "poikanen".
More information (in Finnish) to those interested: Pentuja vai poikasia? (KOTUS)
As a few other people have already stated, what the word "pentu" really means is "a cub" since the species of the animal in question is unspecified. "A kitten" is "kissanpentu" (a cat's cub, literally translated) and "a puppy" is "koiranpentu".
I've translated "pentu" as "a cub" in other exercises and it was accepted. I assume that these issues will get fixed, as this tree is still in beta!
In the Vocabulary section of Tips for the lesson Pets 2 (and other domestic animals). Duolingo specifically states; "pentu can be mean puppy, kitten, cub (in that order). Since the statement is "Sillä on monta pentua". Puppy should be accepted. Maybe to a fluent finnish person, there is a better word for puppy, but we are all here learning via Duolingo. As such, puppies should be accepted