Haiku started life as an x86 only OS, since then it's been partially ported to ARM, PPC and 68k.
Of course, I was referring to BeOS which Haiku is based upon. BeOS ended up going x86 to appeal to a larger market. They had a deal to be shipped as an alternative start up option sitting next to windows on the computers that Dell, Compaq, HP, Toshiba, and the like were selling. Microsoft caught wind of this, and reminded these companies of the fine print on their contract and the rest is history...
I'm sure you all know the story.