Theo is a given name, deriving from the Greek word Theos which means god, short for many different names in which 'Theo-' is the first element, for example:
Thea is a feminine given name of similar origin.
Many other names beginning with "theo" do not necessarily derive from Greek, but rather the old Germanic "theud" meaning "people" or "folk". As these names are not common today and in the ancient times they were not spelled with "theo" it is unlikely this nickname was ever applied to these individuals. These names include:
Read more about Theo: Notable People Named Theo