2001: A Space Odyssey
The extraterrestrial species that built the Monoliths is never described in much detail, but some knowledge of its existence is given to Dave Bowman after he is transported by the stargate to the "cosmic zoo", as detailed in the novels 2001: A Space Odyssey and 2010: Odyssey Two by Arthur C. Clarke. The existence of this species is only hypothesized by the rest of humanity, but it is obvious because the Monolith was immediately identified as an artifact of non-human origin.
During The First Celestial Host on Earth, the Celestials placed the first of many Monoliths in a jungle and exposed it to early Man, who were largely afraid of it due to the high-pitched noises it made. Even though its functions are virtually unknown it is highly possible that The Monolith is a recording device of some sorts as well as a gateway to another dimension, one that tends to evolve any who enter it beyond their natural state of being.