One of the first design spaces that people tend towards is Tribal effects. Design a bunch of creatures, all with the same type, then make a few lords that give all creatures of that type +1/+1, or flying, or double strike...the list goes on. While Tribal effects can be used skillfully to create cohesive environments (Innistrad comes to mind), when a set is inundated with them (Lorwyn or Onslaught, for example), they tend to monopolize the environment in which they're placed.
If I wanted to make a cookie-cutter Pony set, I'd fill it full of boring common Pony creatures, a few uncommon "All Ponies get +1/+1" effects, and a couple silly mythic rare "Whenever you cast a Pony spell, draw nine cards and untap all lands you control" bombs. But that's not the environment I wanted. Yes, there's a lot of Ponies in the set. Yes, they are kind of the entire point of the set. But there are more ways to created cohesive deck ideas than Tribal effects alone.
For example, the Wonderbolts (Soarin', Fleetfoot, Rapidfire, and Captain Spitfire) all share the Wing Power pseudokeyword, a triggered ability that "goes off" whenever the creature itself or another creature with flying you control enters the battlefield. I could have very easily made Wing Power trigger only for Pegasi, but by having it trigger on all flyers instead of a select few, the variety of decks that can be made using Wing Power is far higher, while still allowing an obvious "Pegasus Tribal" deck to exist.
Now, this is not to say that there are no tribal cards in the set whatsoever. The presence of Changelings in the set makes Tribal interactions more interesting, as they count as all creature types at all times. Changelings allowed me to make a few sillier card designs, such as Lyra Heartstrings and Changeling Saboteur. In addition, there are a handful of characters and effects within the setting of Friendship is Magic that just seem too obviously Tribal to ignore; this is the case of Spike, whose character basically exists to provide helpful assistance (and comic relief) to Twilight and her friends. Finally, there are a handful (one or two per color) of common and uncommon cards that have mild Tribal effects, but none of these cards are powerful enough on their own to justify designing an entire deck around their Tribal interactions. Instead, they merely provide a slight bonus, applicable most acutely to Limited players, when their Tribal interaction is turned on.
Long story short; while there are a few Tribal cards in Friendship is Magic: The Gathering, Tribal interactions are not a focal point of the set's design.
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