I think many animals are far more intelligent than we think. The problem is, how do we test it? Their intelligence is far different from ours, and we take problems in communication and their failure in tests designed from a human standpoint as evidence that they are stupid.
Many people seem to have an odd notion in their subconscious that they are the picture of perfection. They would never say so, but they demonstrate it in the fact that the more different any given person, animal, or thing is from them, the worse they think of it.
I'm saying that we think animals are dumb for the same reasons that we practice racism.
Without realizing it, we have an attitude of 'you are different, therefore you are inferior'.
Often it depends on the animal. I have seen a lot of dogs that are really obviously dumb as a post, and others that are very clever.
We have two cats that are brothers and one is quite a bit smarter than the other, based on my observations of their behaviour.
I guess the same can be said of humans though too.
What do we as humans consider intelligent? Different dogs and even dogs within one breed, show different characteristics.
Every dog is "intelligent" in a different way. Some are great at herding other animals, others are very intelligent at sniffing out things or following a trail, others may be great at doing tricks...
You'll find that even the biggest numbskull has something they're really good at! Even if it is just cleaning out a food bowl really thoroughly!