You can easily obtain a secret pet in World of Warcraft. Learn how to get the Dirty Slime here.
In World of Warcraft, some content is hidden behind puzzles or secrets. This also applies to Battle for Azeroth. In the instance Tol Dagor, there is a hidden pet for you, the Dirty Slime.

Dirty Slime from Tol Dagor
To get the Dirty Slime, you need to visit the Tol Dagor instance in mythic mode. You obtain the pet through the item Dirty Bucket.
Here’s how to get the Dirty Slime: The Dirty Bucket can be obtained with some luck from the hidden boss Gol’than the Stinky. Getting to Gol’than is fairly easy.
Gol’than is located at the end of a corridor hidden behind a wall. You need to kill the first boss of the instance, the Sand Queen, and then proceed to the first level of the instance.

Bring Rogues, Blacksmiths, or Inscriptionists! To finally get to Gol’than, you need a rogue in the group or players who can use scrolls of unlocking or Monelite lockpicks.
If you have a lockpicker with you, you can simply go through the second grate right behind the first boss after he is dead. A few groups later, you will be up top. Your goal is located at the prison cells.
Once you reach the top, you need to open the third cell on the right side. There is a hole in the right wall of the cell that you need to go through.

Not an easy fight: At the end of the corridor, you will find a Dirty Bucket that you can use. Click it, and Gol’than will appear.
Gol’than himself is a small black blob, which already looks like the pet later. He has abilities that can disorient you, and you have to play in a very small space with some poison effects. The fight is therefore not easy, and a full group is recommended.

That Gol’than has the Bucket is not guaranteed. So you need some luck. However, it’s not a big detour to get to him. Therefore, obtaining the pet is suitable for both dedicated collectors and casual treasure hunters.
Want to collect even more cool pets in WoW? Here’s how to get Uuna