Translation:Dogs are clever.
I believe it's because the character 는 is a topic marker and makes the statement about dogs in general.
So how can you see when they'd use 1 dog in particular? What's the difference with the plural form?
개 can mean dog or dogs, if you add 들 it specifies that it's a plural. So if you said 'dog' for 개 it should be accepted
The topic marker 는 means that it's talking about dogs in general, if you were to say 개들이 (I think that's proper spelling) you'd be speaking about a specific group of dogs.
Of course the dog is clever. He can dance, write messages, speak Korean, sing, etc.
Plurals don't need to be specifically marked in Korean; whether a noun is singular or plural is almost always inferred through context by the listener/reader. In this kind of context, the use of 는/은 implies that this is a general statement applying to all dogs.