Hello [mention]miltos10[/mention],
sorry for the inconvenience. Do you meet all requirements to obtain the legendary dog item? You neeed, in addition to all legendory dogs seen, also 751 hours playtime and the GS Ball of the quest. If you want to read more about the requirements use this guide: Legendary dog guide.
I hope i could help you out. If you have any questions, feel free to ask! :3