The font of all knowledge, Wikipedia, says that the origin of the term dark horse comes from the race track, a steed that is unknown to gamblers and thus difficult to place betting odds on. The first known use is in a novel published in 1831 by Benjamin Disraeli (!) used in just this context. These days it refers to a little-known person or thing that emerges to prominence, especially in a competition of some sort.
This object is tucked in a corner of Citygarden. It's on a little artificial hill, mostly surrounded by trees and partly hidden. The work is called Zenit (zenith?) by the Italian artist Mimmo Paladino. If you look at it in more light (see first link) it resembles a horse more than anything, but not quite. And what's that sunbursty solid on its back? The zenith? Beats me. The poet Archibald MacLeish said a poem should not mean but be. So go figger it out for yourself.