What does it mean when a pet has 15 energy? (instead of 10)

Pets with Tireless Heart gene have 15 energy instead of 10.Also some of the users may use Aromatic Beans on their pets to give them 15 energy for 7 days.

Tysm for the answer!

Oh thats great, so have much more is a pet with the gene worth? Cuz I was going to sell this specific pet (he is a G3 Lykos)

Also, were does it show when you get a respond in here? Cuz I checked the "Subscribe to new posts in this thread" box but I didnt get notified? Or is there a spc side for notifications?

Even though it's not going to be of much relevance, simply for completion's sake: There was an event a few years back where you could acquire items that you would then put on your pets like a custom skin. These not only looked cool, they also increase the pet's stamina to 15. There were costumes for Dragons, Unicorns and Symurgh. 

If you are curious what they looked like, I have them all in my Spring Time Revelers tab. Just one example would be my Brannen: 

As for your other questions... I don't usually sell my pets so I have no good grasp of pets' worth. And if you checked the subscribe to new posts box, you should get an email notification to the mail address you provided upon signup.. Maybe check if you got it in your spam folder?