system
Given the following post, respond as if you're commenting on Reddit:
human
[My Little Pony] How do surnames work?

In many cases ponies of the same family have the same last name, like Maud and Pinkie Pie.  
The Cake family (Carrot, Cup, Pound and Pumpkin) all have the same surname.  
Rarity doesn't use her last name, but I'm guessing it's less like Cher and more of an image thing. It's plausible it's Rarity Belle.  
So how come Twilight **Sparkle** brother is Shining **Armor**? Is he adopted, or something?
gpt
Good question!

It depends on the family, but surnames aren't exactly a mandatory "thing".

You get the Apples and the immediate members of the Pies who are an incredibly tight knight family, and most of then have either the surname "Pie", but there are also wxceptions. Instead of "apple" as a surname, instead the family has apple themed names (e.g. Braeburn, Big Macintosh, Granny Smith). Then there's, as you mentioned, Mr. and Mrs. Cake, of which it is known that Cup took her husband's surname and the two make a family off of the theme.

What can we deduce from this?

Well, Earth Ponies, it seems, have a much larger focus on families moreso than the other races. This is the race rhat *was*, and still is to some degree, the low/working class. They are the poorest of the three races and many of them have careers in agriculture that are passed down to their offspring. Thus, thier families are likely much more tight knit than those of the pegasus and unicorn races. Choosing themed names or use of a surname such as "Pie" or "Cake" names them as a certified member of the family.

It's likely that Unicorns and Pegasi don't do that as much as their innate abilities isially lend them to be far more individualistic in their life goals and may likely stray away from the family in their lifelong goals.