Originally Posted by
Kio
Meaning if you summon your GC chocobo with a gyshal green, while that is in effect you won't be allowed to call your Unicorn mount to ride. But you will be able to call your same GC chocobo as a mount. If you would want to ride a different mount you would have to use withdraw first. Also, if your GC chocobo is not currently summoned as a companion you'd be able to ride it as a mount as usual.
It's worded a bit confusing, it took me a second look to understand. They probably made it this way for immersion. Most people wouldn't walk their horse by the reigns then jump on a bicycle and ride off leaving the horse to it's own whits to catch up to you later. So it makes sense from a RP standpoint.