This is a mechanic I've been considering suggesting for a little while. Ideally, it won't do anything other than add a marker to a pet.

Pokemon has a similar mechanic to this:

It's just a small bar with shapes that can be used to mark a pokemon based on a person's assigned role of the markers.

I figured it might be helpful for people who do projects (and could mark which pets they want to keep, which pets they intend to release, and which pets they might hold onto for a backup) but it could also be useful for people who hoard. It could be used to mark what they want to sell/breed without setting it publicly, what they want to move while sorting their stables, etc. Being a simple mechanic, people would be able to assign the meaning for any marker for what could suit their needs.

The markings could then be shown as a side bar apart from the image of the actual pet:


I'm not sure if it would be beneficial to show such markings on stable pages, but I would personally place them beneath the name of the pet:


I could believe that the only reason a mechanic like this hasn't been implemented is because it would require holding more information for a pet and would probably be a nightmare to keep track of across the website. But I do feel like it would be useful for people, and thought it might be neat to suggest it.