If you’re having a bit of trouble getting started, perhaps this advice, first published in The Bestiary of Equestria, will help:
The first thing to note when designing a new creature is what role it will serve in your story. Is the creature sentient (can it speak and think like a pony of Equestria)? If so, what motivates it, is it sentient, and if not, how does it survive? There are generally three reasons to add a creature into a game and these are: challenge, story, and flavor. The best creatures provide and mix these three aspects as described below.
Challenge is a very important part of Tails of Equestria and a very important part of the makeup of a creature. A challenging creature may be one that is simply strong or has some combination of talents that makes it hard for the party to face head on. Challenging creatures do not simply have to challenge the party with brawn, however. Often it is better to offer up a social clash or a battle of wits!
Examples from Friendship is Magic: Chimera, The Hooffields and the McColts, Nightmare Moon.
Often, new and exciting creatures will be an important part of your adventures and these creatures will advance the story in interesting ways, giving the players choices as they play. A creature that is important to the story could be one that needs help or one that wishes to harm or hinder the party, but the most important thing about these creatures is that they want to interact in some way with the party and give them a reason to adventure in Equestria.
Examples from Friendship is Magic: Tirek, Princess Cadance, Trixie.
Flavor, when talking about Equestria, is what makes things interesting. It is intriguing and exciting and makes you wonder what might be over the next hill. A creature with flavor is one that tells the players something about the world or simply entertains them. A flavorful creature might be a cheerful shopkeep that strikes up a conversation or it might be a monster that has a peculiar fear of Griffons, making the party ask why and engaging them in the magical world of Equestria.
Examples from Friendship is Magic: Breezies, the Windigos, Discord.
So you have had a think and you know what you want to add intot he game and how they will fit into your story. Now we just need to decide what attributes to give it. You will find that there are a lot of rules that govern how strong or talented a PC can be, but when it comes to creating creatures anything goes. Just remember that you are trying to create an interesting experience and that the only way to win Tails of Equestria is to have fun!
Body, Mind, and Charm
The first thing to decide about your creature is its core attributes: is it strong, clever, or charming (or intimidating!). Here you can choose any die values that you think would make sense. After choosing your stats, you can work out the Stamina of your creature by adding its Body die and its Mind die together.
Talents, Special Abilities, and Quirks
Picking talents gives your creature special abilities that makes it interesting to interact with and can change the course of an encounter in profound ways. Some talents work very well together (such as acidic and engulf) and some give absolutely amazing abilities. It’s up to you how many talents and what dice value talents to give your creature. You may think that some of the abilities you want your creature to have just aren’t covered by any of the talents. This is where special abilities come in. Special abilities are simply little ways that your creature breaks the rules of the game. These are very similar to talents except that they have no die value associated with them and they are usually unique to that creature.
Finally, you must give your creature some quirks – although only PCs can earn Friendship points, quirks help you as a GM keep track of the weaknesses or oddities your creature has.