First of all: Yes, this is because of MLP.

Do ponies ever express as shapeshifters? What about unicorns/pegasi?

I feel like Running Wild did include unicorns, but I don't remember pegasi. But I don't have the book with me at the moment.

I have no idea what MLP means.
However, I do not remember horse shifters in any official source. There may be a weird Mulvihill SR3 adventure on the SR-homepage though.
In regards to pegasi / unicorns: I don't think, shifters ever express as paracritters.

RC says any animal can potentially express as a shifter, but it says nothing against horses doing it. The same with paracritters, it just says they get a Magic 1 for free.

Talk with Bull, he is a strong supporter of the Brony movement. That said, if a Pony does become a shifter, he must learn to flow.

Really? I thought he was against it.

Another victory for the herd >u>

Bannockburn is referring to "They Shoot Wild Horses", a one-shot adventure written for 3rd edition that's available on the Shadowrun4.com website HERE.

The premise is that a Unicorn shaman hires the group to protect a grove while some like-minded shamen perform a fertility ritual. In said ritual the participants shapechange into unicorns, although it's implied that the magic of the ritual changes them as opposed to being shapeshifters naturally.

I believe ponies can express as shapeshifters, though I haven't seen any suggested stats on them. From some quick reading about shapeshifters all I see are that they are an awakened form of a mundane critter, and with unicorns & pegasi being magical critters right off (they have magic attributes listed in RW, 120 & 121), then I don't think they can express as shapeshifters.

If you really want a shapeshifter unicorn, you can always make a herd of ponies that SURGEd with a Goring Horn (see RC, page 114) as a collective racial trait, and are otherwise mundane except for any shapeshifters you want.

ShadowBronies scare me.

That's all I read. I'm out.

Your contempt is noted, and quietly giggled at.

I was under the impression that shifters had to be of a mundane critter, rather than a paracritter. I believe a SURGE variant horse would be in keeping with this, but I am not certain, so please let me know.

Also, since there seems to be a silly confusion, MLP is My Little Pony, yes? As for Neraph, he must not be familiar with My Little Wu-Tang Clan. I shudder to think how many times someone watched that cartoon to lipsync like that. But you HAVE to see that video at least once in your life.