Horizons wasn't in Epcot on opening day, but it was in the original plans. Epcot opened October 1, 1982. The original plans called for Horizons to open one year later (which it did) and The Living Seas in 1984 (which was late). There were also original plans for a Space pavilion and a Life & Health pavilion in Future World, along with signs in World Showcase for countries "coming soon" (I can remember Israel and Spain, but there were others).
The book Walt Disney's EPCOT contains an artist rendering of Epcot center with Horizons in a different location and with a totally different building design. Regardless, this was my all-time favorite Disney attraction, and I still miss it.