I read on here that it can be found in Oerba AF300 near the big tree with all the roots sticking out, to ensure I'm in the right spot, if you were to start from the main entrance gate, go north past the artefact gate, and to the next right hand corner. If that's the right spot, then I must be doing something wrong, all I'm getting are ghasts and pink lilies, I have battle mania on, if that matters --- "Apparently you can have your desire and still suffer terribly for it, a lesson for all who would consider marriage" ~ Morrigan - Dragon Age:Origins
I had it spawn for me near the original entrance. Like up towards the crystal mounds.
I also heard someone on here say that chocobos aren't actually classed as 'rare'. I was hunting the purple chocobo and kept getting the tonberry spawn with Battlemania on, so I tried turning it off and the chocobo did spawn more regularly. Might be worth trying, can't really verify that though.