Yeah, that's Mr Hands! I could never stomach it (it's on the short-list of "Shit I Won't Watch", right alongside the various Afghani decapitation clips and everything done by Luka Magnotta) but a friend of mine watched it, and said it was really funny. You can still find the clip pretty easily by searching for "Mr Hands".
(now that you mention it, audio clips from Mr Hands WOULD be pretty handy for my upcoming multimedia project, Equus! The Musical - a lighthearted musical version of Equus starring the ponies from MLP and an emotionally-unbalanced brony whose clop-fetish drives him to commit unspeakable acts of violence)
And garcet, yeah, OK, when you put it like THAT, it does sound a bit weird. But I stand by my theory; the scientifically formulated cuteness and un-hoselike qualities of your typical cartoon pony override the typical clopper's cultural inhibitions against jerking to horses. And let's face it, real horses are fucking scary-ass creatures (they are one of only two animals I am absolutely terrified of) so there's that to consider, too.
Also, I'd just like to point out that none of the Mane Six are "underage". To date, neither primary nor secondary canon has established an exact age for any of the Mane Six, but having studied the matter at some length, I can say with confidence that the ponies are all twenty-somethings. All of the ponies are fully responsible adults who live independently (with a possible exception in Pinkie Pie, but I won't get into that now, as it'd take far too long) What's more, five of the six have already entered the workforce - AJ as a farmer, Rarity as a fashion designer, Pinkei Pie as a Michael Alig-style party planner, Rainbow as a HAARP perp, and Fluttershy as a professional animal hoarder. Only one pony is still in school - Twilight Sparkle - and she is quite clearly a grad student, possibly even a doctoral candidate (or whatever equivalent degree magic unicorns get in the Equestrian academic world).
Now, there are THREE slight niggles in all this. I'm sure some of you are just dying to call me on them, so I shall address them now and get it over with: the first problem is their apparent age in Equestria Girls. The human-ponies in this film are quite clearly highschool jailbait, so, technically, yes, the Equestria Girls are underage. However, the human versions of the Mane Five have only a tenuous connection to the pony Mane Five, and it would be a mistake to infer too much about the ponies from their human counterparts. I guess this brings us to an interesting dilemma: is it more fucked up to clop to of-age ponies, or to fap to underage humans? What does our choice say about us? I do not have an answer for that question at this time.
The second issue, which is easily dismissed, relates to the ponies' aforementioned neotenous physiology. At first glance, one might expect a "grown" pony to lose her cute, childlike traits and become more like a fully adult horse. However, this is not the case at all; Cheerilee, Mister and Mrs Cake, even Granny Smith, all display a remarkable degree of paedomorphosis, despite their obvious age. The ONLY ponies who become horse-like are pony royalty and, to lesser degree, pony alpha males, and this horse-ness does not seem to be dependent upon age... at least not from the time of adolescence (it is possible that certain horse-ponies were more pony-like back when they were still blank flanks; evidence has not been forthcoming, but one hopes that this matter will be cleared up once and for all in Season Four, as we observe what happens to Twilight's physiology now that she is a full-fledged alicorn princess)
The third issue, which I will admit MAY have some validity, is related to Equestria's social order and technological development. In our real-world modern society, human beings experience a "long childhood" - we do not "mature" to the point of independence until we're in our late teens and early twenties (some of us never do). However, this long childhood is a relatively new phenomenon, brought about within the last couple hundred years or so, and is mainly the result of industrial child labor laws and the growth of compulsory secondary education.
Now, pony society is still largely pre-industrial and agrarian. It is ruled as feudal-theocratic kingdom, with certain concessions to democratic rule on a local level, and, while there are (presumably state-funded) schools for adolescent fillies and colts, Equestria does not appear to have embraced either child labor regulations (Applebloom, Sweetie Belle) or the concept of "high school" (Twilight has no idea what this is). So, it's *possible* that the Mane Six are, in fact, teenage jailbait, albeit the sort of medieval jailbait that was, comparatively speaking, already well into middle age.
However, I don't believe this is the case, and I cite as evidence the Mane Six's pony libidos, or rather, their lack thereof. Contrary to what the clop community seems to believe, none of the Mane Six are particularly sexual or hormonal. (here I would like to refer you all to the video entitled "Romantic Love (or the lack of) in MLP", by Youtube pegasister, Clover Keen) If the Mane Six were teenagers, fresh from the jaws of puberty, we'd except to see a lot more drama, cattiness, boy-chasing, etc etc. Instead, none of the ponies seem to be much bothered by their biological need to get fucked raw. Rainbow and Fluttershy are, of course, lesbians together (this is canon. I will fight you if you say otherwise) but by and large, they come off as early-to-mid twenty-somethings whose hormones are leveling off. So, NOT underage horses!
Of course, you might have been referring to CMC fillycon fans, and if that's the case, yeah, the CMC are totally underage.