Story milestones in Super Animal Royale are hidden challenges that reward you with certain cosmetics when you complete them. There are some hints to completing them, but often you need to explore the environment to find the solution. After finding the Secret Lab Lab, you are told of the first created Skullcat being nearby. Here is how to find it and complete the story milestone called The First Super Skullcat.
To complete The First Super Skullcat, you need to find the location of this ancient being. When you talk to Dr. Dogna in the Secret Lab Lab, she tells you that the Skullcat should be located nearby in the desert land. She is correct in that statement, and you can find it in the same game that you unlock that previous story milestone.
The Super Skullcat is located just to the south of the Super Pyramid in the C4 square on the map. There is a sphinx that you will come across, but you will need to open it up. Go to the left side of the sphinx, and you will find a pressure plate. Stand on it to open a door so you can now enter the sphinx.
Inside, you will find various loot you can grab, and the Super Skullcat covered in mummy wrap. Talk to him, and he will attempt to scare you before giving you a new cosmetic for your melee weapon. You can unlock it by redeeming it in the milestone tab on the main menu. If anyone talks to him before you, he will not reward you, and you will need to start over in a different match.