Hepta-

Hepta- was an old prefix proposed a few years before Zetta-, it comes from Greek meaning 7, as it was the 7th power of 1000. It was later removed and replaced with Zetta- beacuse it's symbol, h was already used as a symbol.

The reciprocal of this prefix is septo- from Latin septem meaning 7. It was also dropped and replaced with Zepto- because it's symbol, s was also already used as a symbol.