Animal intelligence being fundamentally different from human intelligence.
This is not an accepted scientific fact, but it is a commonly assumed idea. While it has been mostly abandoned/made irrelevant among people who do research in intelligence and neurology, I find that this sort of idea is very common in most lay conversations about intelligence. We are going to come to understand the common neurological basis of intelligence very soon and then we will very quickly see how the brain of other species are different neurological arrangements that directly translate into superior performance for tasks particularly important the ecological niche that those animals occupy. It is just a difference of configuration crossed with a difference in quantity, but configuration will matter much more than people think it does now. We are not super geniuses with enormous brains. We are animals with slightly larger brains that are configured to do a set of tasks that are particularly useful and, importantly, self-improving via transmission of knowledge across generations.
This will mean that we will be better at picking out common neurological verbs between species. Attributing human-like behavior to animals ("My dog is sad" "My dog is being generous") will seem less like a pathetic fallacy. It should not seem like a pathetic fallacy anyway (except in ridiculous cases), as after all we are mammals so if we are capable of a neurological phenomenon -- a thought -- then there is absolutely no reason that another animal with a brain cannot have it as well, given the right configuration. And we know we have common brain structures with dogs and cats.
It will also become obvious that the consequences of intelligence are as much dictated by physiology as by neurology. There will be recognized a critical learning and evolutionary feedback loop between the brain and the body. If you evolve an opposable thumb, you will evolve a brain that can use a hand with an opposable thumb. That is: the physiological ability to manipulate the world will lead to the evolution of a brain capable of complex reasoning about the state of the world and thus better able to manipulate it. Mutually, such an improved brain will increase the potential fitness of an even better hand/thumb, etc. This is recognized in the evolution of limbs in general at present: the ability to reach out with a limb and pick up food was a huge evolutionary achievement that required much time and the aforementioned feedback loop. Observe animals with more primitive brain structures, lizards and turtles, for example, and you see that if they see something they want to eat, they will move their heads/eyes/brain toward the food. Animals that have had the advantage of a physiology/cognition loop to develop the ability to reach do not do this. They reach. This seems like a tautology, and it is, but the physiological requirements to move a limb separate from your brain/eyes and coordinate its movement through the world are significant.
tl;dr T Rex had short arms because he didn't have a brain that could do the computation required to move a limb in a separate location from his eyes, mouth, and brain.