How did the half-ponies/pony-people get ears and tails?

[HorsesandMOARGaloar 300]

From what scene or ability do the Mane Six/plus Sun-Shimmie get those?

[Andaasonsan 3,205]

In the first movie they got them when they accessed the Magic of Friendship.

 

In the shorts they seem to get them when they "rock out" on whatever instrument they play.

 

So basically, magic.

 

Oh, and Sunset Shimmer never gets "ponified".

Edited April 29, 2014 by Andaasonsan

[Dattebayo 2,850]

In the first movie they got them when they accessed the Magic of Friendship.

 

In the shorts they seem to get them when they "rock out" on whatever instrument they play.

 

So basically, magic.

The godamn magic of friendship is ruining my day

[Moost 164]

In the lamest freaking possible way that doesn't make sense. They look like fucking Mary Sue's when the get the magic.

[Hive King 2,001]

I don't know about RR, but the best answer is that the Elements are from Equestria, therefore, their users fuse with their equestrian selves a bit

[Frith is Magick 1,471]

With the power of imagination

No, it isn't actually imagination....

 

It's magic

 

Magic (n): Don't feckin' question how it bloody works

[Scoots and Dash 208]

Obviously with by using... Magic

 

 

And to answer your question of what scene: They get it at the end via a sequence of awesome events. I don't really wanna spoil it if you haven't seen it or for anyone who has but I'll say so in a SPOILER below:

 

 

Showdown in the courtyard

 

Sunset appears to give up, throwing the sledgehammer down. Twilight's friends surround her and commend her selflessness. Hearing the flurry of kind words directed towards Twilight, Sunset loses her temper and charges at Twilight, tackling her and knocking the crown off her head. During the minutes that follow, the crown repeatedly trades hands as Twilight and her friends attempt to keep it out of Sunset's, Snips', and Snails' possession. In a moment of absentmindedness, Twilight tosses the crown away, and it ends up in Sunset's hands.Twilight and her friends find themselves face-to-face with Sunset Shimmer, brandishing a sledgehammer and ready to destroy the portal back to Equestria. Twilight tells Sunset not to hurt Spike; Sunset, saying she "wouldn't dream of it", tells Snips and Snails to let Spike go. Sunset then gives Twilight an ultimatum: give the crown back and be allowed to return to Equestria, or keep the crown and never go back. Twilight, maintaining her fortitude, keeps the crown. Surprised by this, Sunset tries to reason with Twilight to hand over the crown, to which Twilight replies by explaining she's seen what Sunset has been able to do to Canterlot High even without the magic the crown will give her.

With the student body under her control, Sunset explains to Twilight that it was never her intention to destroy the portal; she wanted to crown not to rule the school, but to rule Equestria, with the help of her brainwashed followers. Twilight stands her ground however, and her friends back her up. Unfazed by their defiance, Sunset hurls a fireball at Twilight to take her out. Just before the spell hits, Twilight's friends huddle around her.A giddy Sunset immediately places the crown on her head. As soon as she does this, a magical aura surrounds her, and she's lifted up into the air. Twilight, her friends, and onlooking Fall Formal attendees watch in horror as Sunset is surrounded by dark flames and transformed into a demonic creature, with red skin, fiery hair, wings, and a tail. Using her magic, Sunset proceeds to turn Snips and Snails into creatures similar in appearance. The students retreat to the supposed safety of the school doors, but the transformed Sunset tears away the entire front of the school, declaring herself the students' princess and demanding their loyalty. She flies into the school and projects her magic onto the fleeing students, hypnotizing them into submission.

Sunset cackles at her apparent victory, but is shocked to find her attack had no effect. A pink force field surrounding Twilight and her friends had protected them from Sunset's attack. Twilight reasons that the magic contained in her Element of Harmony united with "those that helped create it". An aura from the crown engulfs Twilight and her friends in light, linking each of them to their Element of Harmony and giving them pony-like traits, including actual ears, wings, and hair extensions resembling tails when viewed from the front. Twilight finally proclaims that while Sunset may have the crown on her head, she cannot wield its power because she lacks the most powerful magic of all: the magic of friendship.

The magic surrounding Twilight and her friends explodes, causing a double-helical rainbow to shoot out and engulf Sunset Shimmer in a rainbow-colored tornado that strips her of her power and releases the students of her mind control.

 

Aftermath

Twilight tells Sunset that she will never be able to rule in Equestria and that she has shown everyone who she really is. Sunset, reduced to tears, apologizes to everyone for what she's done. Twilight goes on to say that the magic of friendship exists everywhere, not just in Equestria, and that Sunset can either seek it out or "forever be alone". Sunset explains she doesn't know anything about friendship, to which Twilight points to her friends and says they can teach her.As the bright light dissipates, the students congregate at the front of the school. Spike runs out to Twilight, who is lying on the ground with her friends. Twilight picks up Spike and hugs him while their friends approach her. The seven of them notice a large crater in the ground, with Sunset Shimmer—returned to normal—lying at the bottom of it.

As Spike cheers the girls on, Principal Celestia walks up to give Twilight her crown back, announcing that what everyone had seen that night was exemplary of how a princess should act. Flash Sentry then extends his hand to Twilight, asking her for a dance. The scene cuts to the auditorium once more, with the students dancing to a reprise of "This is Our Big Night". Flash Sentry joins Twilight in a very equine dance style before Photo Finish takes a group picture of Twilight, her friends, and Spike.

All information is straight from: The MLP Wikia Page

[Courageous Thunder Dash 7,824]

Magic, specifically, the elements of harmony. 

[Larkybird~ 734]

Magic. How else would they have gotten them? Well Magic is just the simple answer and Scoots and Dash gave the full answer.

[Scootalove 10,689]

Magic, more specifically: The Elements of Harmony.

[ponyvangelist 732]

Scoff if you must, but I have a theory...

 

[Wind Chaser 4,768]

Everypony just saying "magic", smh.

 

I'd say a specific kind of magic, one that propagates a connection between both worlds. One can see that in the fact that the Elements of Harmony worked in both worlds (with the BS Deus Ex Machina notwithstanding) and the parallels between Rainbow Power and Rainbow Rocks.

[tripfly 264]

Also they don't get tails, their hair just grows really long and looks like a tail