I don’t really see any race in any of them, so I’d just roll a die and pick at random.

@Princess Tuna  
Because Speedy Gonzales.

@Elusive Badgerpony  
Ayy nigga don’t be so afraid to ask questions. There’s a reason this pic looks off. Allow the Mighty Niggertron to explain.
 
@Kryptid  
Voice is really all we got to go on when guessing ethnicities for humanizations. It’s for that very reason you’ll see images of Sapphire Shores as black even tho she ain’t a zebra. Take a guess what her VA look like.  
Cross-ethnic voice acting does happen frequently as you say, but that’s cuz VAs is good at making themselves sound like different races. Take this nigga for instance:  
 
If you didn’t know, his VA is Tara Strong, but it worked because she was able to sound like a lil yellow niglet. If she hadn’t it would have been as jarring as the guy from this scene:  
 
This pic is similarly jarring as the voices and races don’t seem to match up.

Ignore these idiots. This picture is lovely.

@Elusive Badgerpony  
Nothing about Twilight screams any human race (except, arguably, her voice, since she is voiced by Tara Strong. That’s not terribly relevant though, since cross-racial voice acting happens rather often). Race is not the same thing as behavior or culture and should not be conflated with such.

@soundtea  
With such a simple pose?  

The biggest hole in this kind of thing is what are the odds that an entire human mane six would all be different ehtinicities and live in the same area, unless they were intentionally seeking each other out to fill some form of bizarre friendship race quota?
 
Diversity isn’t a bad thing on its own but it just bugs me when people ignore that humans through most of history and currently live in areas that are heavily dominated by only one, two, or perhaps three ethnic/racial groups so it can look weird if you make several characters all have different backgrounds.

rainbow dash is always the Mexican one.
 
i wonder why.

I don’t mean to start any shit but I don’t really get making Twilight Sparkle African-American in these humanized things. It’s not a thing that bothers me so much as it’s a thing that kind of confuses and bewilders me.
 
I mean, these are pastel-colored ponies in the source material so go wild with whatever races please you, but nothing about Twilight really screams “African-American” to me. I am just some cis white guy though so maybe I’m just inherently racist or something but I don’t understand the purpose behind assigning a character to a minority race for, seemingly, little purpose besides an illusion of diversity.
 
I am very interested to hear justifications for the “Twilight Sparkle is African American” thing that isn’t “because diversity is good”. It is, but when it’s achieved by simply plopping a character without any defining racial characteristics into brown skin and then not giving their race any weight or importance to the way they act or behave is just as meaningless and racist as color-correcting Mane Six headcanon pics so that all the characters are white. It’s erasing the culture that these characters would have grown up in. It destroys the importance of race by eliminating what makes those races interesting and unique, as opposed to supporting the coexistence of races together in s give-and-take of cultural traits and quirks based in coexistence, peace, and mutural love.
 
Look at me, getting all ranty about humanized headcanons. Again, they’re pastel-colored ponies in the source material, so you could pretty much make them all any race. I’m just curious as to what prompted you to select the races that you chose.

I thought Caesar Zeppeli was in the thumbnail for a sec.