Similarly as with Gold Poring and other pets’ final skins, in order to get the Gold Lunatic skin for your pet unlocked, first of all you need to reach Intimacy Level 10. Remember that the change is purely cosmetic and no stats boost occurs upon unlocking.
In my opinion the fastest and cheapest way is to purchase a few Rainbow Carrots through Exchange and to gift them to your Lunatic (click on pet’s icon and choose Gift). Rainbow Carrot is 10-13k zeny/each and you will need 6-10 of them to get Lunatic from intimacy level 1 to level 10.
If you prefer to do it the cost-less way, please check the guide below.
Check: How to increase Pet Intimacy
When your Lunatic is intimacy level 10, click on its icon (it’s in the top-left corner, below the character’s icon), then Check. The pop-up with its stats will show up. Click on Lunatic avatar in bottom-left corner.
You will be able to choose your pet’s new skin!
That’s all! Enjoy your pet’s new look.