Rare Materia Checklist  
FF7  
http://steamcommunity.com/sharedfiles/filedetails/?id=159036331#122916
Choco/Mog - Say 'Wark!' to one of the Chocobo's at the ranch to get the Choco/Mog summon. This materia is available until you are able to breed your own chocobos, and the ranch is cleared -- which is sometime during the Huge materia quests.

*You can however respawn this materia. Once you capture a chocobo, it will appear in the farm. Talk to the chocobo and you will receive the materia.