Applejack is probably the easiest card to design flavorwise.
I wanted to make conceptual Magic: The Gathering cards based on the characters of the My Little Pony: Friendship Is Magic TV show, with these guiding principles:
Here are the cards I have created so far (more to come in the future), with the ideas behind them explained.
Applejack is probably the easiest card to design flavorwise.
Green is the obvious choice of color for an Earth pony (who also happens to be a farmer.)
Ponies are - relatively speaking - quite weak, so having anything larger than 1 or 2 for power/toughness would be unrealistic flavorwise. Applejack packs quite a kick, so a power of 2 is probably justified.
Applejack could be just a vanilla 2/1 creature, but that would be a bit boring (and also difficult to balance in terms of mana cost). Considering that she's a farmer, it actually makes sense that she could fetch a forest card on entering the battlefield.
(This makes it similar to Viridian
Emissary, except that it makes sense for Applejack to fetch the land when
entering the battlefield. Also, as a farmer, it makes most sense if she
fetches a forest rather than any basic land card. While this is rare, it's
not unheard of. For example Silverglade
Elemental has the same ability.)
Rainbow Dash is a pegasus pony, so in MTG terms the card has to be white
(because that's the color of most/all pegasi in MTG).
Flying is an obvious static ability.
What's much less obvious is that Rainbow Dash is really fast, so the card should really have some other ability that's related to speed.
Haste would be the most obvious choice, but it's dubious in terms of MTG flavor because haste is almost exclusively used for red creatures. As far as I know there's only one single white creature in the entirety of MTG that has haste (namely Akroma, Angel of Wrath), but it's a unique exception, so I'm not going to break this rule here. (Flavorwise one could argue that Rainbow Dash might be fast in terms of flying speed, but not necessarily fast to react when summoned, so no haste.)
Another ability that's also related to speed and which happens to be very
common in white cards is first strike, so that was the second most
obvious choice. (This also makes more sense in terms of flavor because fast
flying speed can be argued to allow first strike.)
Blue is the most obvious color for unicorn ponies because they deal with
magic.
What kind of magic ability should she have?
In the show, she's a bookworm who has a large library and is constantly reading and checking books for spells. In MTG this notion is translated to handling your library (ie. your deck when it's used in play).
Drawing cards and scrying are two common abilities related to this, and also very typical abilities of blue creatures, so they were obvious choices.
Scrying is a useful but not overly powerful ability in MTG, so I gave her the same ability (for the same casting cost) as Augury Owl.
However, she has no flying, which would make her more expensive than Augury Owl in terms of mana cost. Also, I wanted her to have an ability related to drawing cards as well, which adds to the flavor.
Now, drawing cards is a very powerful ability in MTG so it should not be
given cheaply or else the card easily becomes too powerful and unbalanced.
I copied her card drawing ability cost from Azure
Mage. Now Twilight Sparkle is almost the same as Azure Mage, except that
instead of having a power of 2, she has scry 3 on entering the
battlefield (which makes more sense flavorwise and should keep the card pretty
balanced).
Now this is a tough one.
Rarity is a fashion designer and dressmaker. Like all unicorns, she has magic powers, but she uses them primarily for dressmaking. This doesn't translate very well to anything significant in MTG.
Another thing that she uses her powers for is finding gems. While there isn't anything really resembling gems in MTG, this is, however, the closest thing I could think of that could add flavor to the card. I just changed gems to (valuable) artifacts. While it's a bit far-fetched, it's not all that off either.
The card copies largely Treasure Mage. However, 2/2 would be too much for a pony, so instead I made her a 2/1 and lowered the minimum mana cost of the searched artifact.
Rarity is surprisingly tough at fighting, matching even Applejack (as seen
in several episodes). Hence a power of 2 is justified.
This is also a difficult one because Fluttershy doesn't have any
characteristics that would have a very obvious counterpart in MTG.
She's a friend of all animals (except dragons), so that's one direction that can be taken. However, there are many, many abilities in MTG that could be seen as describing this, so it's hard to choose. (I could have also chosen to make her ability somehow related to her fear of dragons, but that seemed too specific and narrow.)
In the end, I decided to copy the ability of Adarkar Valkyrie. The rationale is that when a creature dies, Fluttershy nurses it back to health, after which it becomes her ally.
The mana cost of this card was hard to decide because there's no direct
comparison to any very similar existing card, so I went with my gut feeling.
Princess Celestia is a pegasus and unicorn, hence a white/blue card, and
flying. Also she has a legendary status in the show's universe, thus it
makes sense to make the card likewise legendary from a flavor perspective.
Princess Celestia doesn't do a lot in the show, so it's not trivial to come up with flavorful abilities.
Two things she has been shown to do is to undo spells (hence the obvious parallel to countering a spell in MTG) and to shoot a solar beam (which translates quite well into dealing damage to a target creature).
The spell countering ability is taken from Ertai, Wizard Adept, and the damage to target attacking or blocking creature from Ballista Squad.
Flavorwise she could also be considered to be an inspiration to ponies, and thus a boost in fighting spirit.
Another situation where it was difficult to determine a balanced mana cost
for the card, and thus once again I went with my gut feeling.
This was a really interesting card to design because the character of Princess
Luna translates extremely well to MTG flavor.
Princess Luna is Princess Celestia's younger sister who turned evil because she resented that nobody appreciated the beauty of the night (which she was in charge of). Celestia banished her, but a long time later she returned and tried to impose a perpetual night on Equestria, but was defeated, and her good side once again prevailed, although her evil side still shows hints of existing.
Flavorwise it makes sense to say that her "blue magic" turned into black magic, hence the white/black coloring. This white/black color contrast also reflects well the internal struggle between good and evil she fights with.
Her good and evil sides have different personas (Princess Luna vs. Nightmare Moon), and she literally transforms between the two when she lets her evil side take hold. This transformation translates really well into a transforming MTG card.
She can control the weather, and has used this to, among other things, raise a mist that stopped other characters from finding each other. This resembles Fog. I gave her a similar white ability, copied from Benalish Missionary.
Nightmare Moon instilling fear is also one of her key traits in the show, and also this translates extremely well (and literally) into MTG.
Nightmare Moon has also been shown to be able to control (or at least affect)
other creatures, hence I gave her the ability of Infernal
Denizen. (The idea is that if she transforms back to Princess Luna, the
player loses control of that creature. If this were a real card, it would
probably require such a ruling clarification.)