Pets are very important throughout my HOTL3 experience so far. I would go as far as saying, I wouldn't have made it so far without them. So every newbie should know that pets is a very good way to help in tanking and putting in some damage themselves. A higher gen player may scorn the usage of pets but for a 300 HP Gen 1 Smurf when I first started, a pet was indispensable.
This post will be updated again when I have more information. These will not be easy to find because I only found them by chance. At the moment, I am looking for a level 30 pet because as you can see, the gap between wolf (level 25) and super flying cloud (level 35) is a good 10 levels. I found that I struggle a bit as a low gen player because of the gap.
Regrettably, I didn't put in enough effort to compile the interesting traits of pets. Such as a Horse can do a cyclone kick, a Green Dragon can do a bard-like Dragon song and the Super Flying Cloud sometimes wander off by itself to another room etc. Perhaps I will do that in future. So this may be work-in-progress for now.