You can use Power Up to increase your Animal's ability.
If you Power Up your Animal, the Animal's attacking power and the number of Stones increase.

Number of times you can Power Up
2 Star Grade: 4 times
3 Star Grade: 6 times
4 Star Grade: 8 times
5 Star Grade: 10 times

※ You cannot increase an Animal's HP using Power Up.
If you want to increase your Animal's HP, you need to draw more of the same Animal.
The more of the same Animals you have, the more their HP will increase.