It depends on what talents develop when you train it.
If it's the old Nightmare that was a hybrid, it may not even have the possibility of healing you. The old Nightmare has to be hatched with some other pet that has "Spritely" and then it has to develop that talent when you train it.
The NEW Nightmare that you can buy with crowns has the possibility of getting "Unicorn", which seems to be a new talent that "may cast Unicorn". According to the petnome project, the (new) Nightmare is the only pet that has this talent. But what a sweeeet talent, healing for your whole team!
Some people say that if your pet is going to get Spritely, it will get it at teen. But that's not right, I have pets that got Spritely at Adult and Epic.
Some say that what talents your pet will get, and in what order are determined when the egg is created. Some say it depends on what games you play and what you score at the games. Others say it depends on what & how you feed your pet and that you should try to keep all the progress bars as even as possible to get the best talents. I don't believe any of them, I like to think of it as random.
I just got a nightmare and trained it to adult and it developed Unicorn.
However, it has never cast it - either in pvp or in normal game battles.
Can anyone tell me the probability of the unicorn spell being cast? Can breeding help improve the chances of casting? Can additional pet training do it?