You're making a false assumption. Humans evolved ever-increasing intelligence to compensate for other areas that we were lacking in physically. We didn't have claws or tusks, so the humans that learned to use spears other tools were more able to adapt to threats by creatures that had better natural armaments. Other animals were already largely adapted to their environments, and did not require such drastic cognitive abilities to cope with the areas in which they evolved. Brain mass is an enormous energy expenditure, but would provide far less initial benefit to many other creatures than it would to humans. You could make a tiger smart enough to use tools, or language, for instance, but they lack both the physical adaptations to successfully utilize most tools and the social structures that would be best served by complex language. The physical adaptations and social structures of the great apes left them in a unique position to benefit greatly from increased brain mass. And there were other intelligent creatures- the ancestors and relatives of humanity, many of which either died off or were directly killed by us. We are all that remains of our branch of the genetic tree, and are also the first animals to have such a high level of intelligence and cognition.
Now that we exist, no such other group will likely ever evolve, so long as we are present. We have disrupted the environment to such an extent that the greatest survival advantage a species can have is its ability to either avoid us or adapt to our presence. Creatures of the size required to develop complex intelligence are quite poor at both, and are often our favorite things to hunt (big cats, whales, bears, gorillas). Even if we were to leave an area completely untouched by human influence, our species would probably be wiped out before we saw another intelligent species. The reason is pretty easy to understand. It took billions of years for one intelligent species to result, as evolution does not select for the smartest creatures, merely those best suited for survival, so we basically resulted entirely by the chance occurrence that one of our ancestors developed intelligence and brain mass in such a time and place that it actually provided them with a survival advantage. Cockroaches, for instance, aren't very smart, but their ancestors will probably far outlive those of humanity due to their resilience and the speed at which they breed. There is no reason for them to become any smarter- a more intelligent cockroach would require more food, and would thus deplete resources in a given area faster. The survival advantage offered by the additional intelligence might save an individual cockroach here and there, but at a significant resource requirement that could instead be used to just breed a larger population of dumb cockroaches. Thus, unintelligent cockroaches are more fit to produce a larger population of offspring, leaving it unlikely we'll ever have super intelligent cockroach overlords.
Intelligence, in an evolutionary sense, is not all that special. Creatures are adapted to their environments, and the way their brain functions must be optimal for the form that they have developed to cope with said environment. Intelligence isn't going to really help a salmon, a sea otter, a desert lizard, or a wasp more than the coloric and homeostatic requirements of such a complex system of neurons hurts them. They're better off saving that energy for sex and maintaining their core temperature.
Also those fish still exist. They aren't some mythical creature like giant men we've never found skeletons of.
http://en.wikipedia.org/wiki/Lungfish
