Bravo, you have given the right answer !! It's a fire hydrant !!
I was really surprised by the great number of hydrants existing in the streets of Chicago. Some of them are even beauties.
Here is an explanation:
In 1871, the Great Chicago Fire broke out, destroying an area of about 4 miles long and 1 mile wide, a large section of the city at the time. Much of the city, including railroads and stockyards, survived intact, and from the ruins of the previous wooden structures arose more modern constructions of steel and stone which would set the precedent for worldwide construction. During its rebuilding period, Chicago constructed the world's first skyscraper in 1885, using steel-skeleton construction.