The name Hayo(m) could be derived from the Old German name Hag-, which means something like; a house or place surrounded by a hedge or fence. Also it could be a latinized form of Gajus.
Anyway, Haio(m), Haie(m), or other variant forms of Haie, like Heike (m/v) or Heitse(m), are still quite common in Friesland today. Therefore I assume that Hayo's probably a Frisian name, although not much spelled with a Y. Also, I don't think that Hayo is a nickname or a diminutive form of Haie, but just another variant. (And because Friesland is a province of The Netherlands, these names are in-fact Dutch names too).