Guild

About
The Guild is a tall, red castle which boasts rewards (Pets) once a certain milestone is reached. Every single milestone that you advance to and complete, a better pet is rewarded (for example, reaching 3.5 billion total strength earns you the Royal Blue Slime.) There are two types of milestones, total strength and eggs hatched. The first pets aren't shiny, but when you reach the 6th pet (strength, 2 trillion total) and when you reach the 4th pet (eggs, 5,000 hatched), this and all the following pets are also shiny.

Trivia

 * The Guild is known as the "easy way of getting pets", and for a good reason. This is because some pets obtained from this can be so insanely overpowered, that people don't need to grind for the legendary pets.
 * During the Ancient Update, you were able to claim already claimed pets once more, from both the strength and eggs section.