Scientific findings that squirrels are intelligent animals sounded nutty at first, but now that doctors are encouraging people to eat more nuts because they're such healthy food, those squirrels are looking pretty smart. Pigs are considered intelligent too, and they eat garbage.
People and computers are intelligent when they show a high level of understanding. Intelligent comes from the Latin "to understand, to gather," and intelligent creatures gather and make sense of information easily. Many people who aren't labeled intelligent show creativity with resources, or an intelligent use of space, money, or time. While not everyone is considered intelligent — there are even smart vs. dumb computers — most do things that are intelligent, just as intelligent people can do things that are dumb.