Q#1: Who decided that having two horns (example, a devil) was a Bad Thing, but having only one horn (example, a unicorn) was an okay Good Thing? and Q#2: What happened to all the older vampires (even over 30)? Buffy seems the only one out. Did the youngins eat the old ones?
-- Jan, downtown
The Two Bad Horns theory seems to have been developed by -- you! Granted, if devils are depicted with horns, they have two of them. But pictures of Moses and Pan and Elsie the Cow show two horns, so horny things aren't always bad. And unicorns have a very fierce side to their fabled personalities, so there you go.
Horns in whatever number have traditionally been symbols of power, for good or evil. Our Western Satan and his devilish minions haven't had a definite iconography, so through the centuries they've been depicted as hairy, hairless, horned, hornless, with bat wings, with angel wings, with no wings, tailed, tailless, all red, all black, naked, clothed, with hooves, or with feet. But whatever else he has, the devil rarely has genitals. Go figure. If the two-horned model predominates today, it's from biblical translations that call "hairy ones" or "goats" servants of the devil.
If two horns are powerful, one horn is really, really powerful. It's thought to combine the power of two horns into one. (There's actually some substance to that idea, since a single horn in mid-face is a more efficient weapon than one horn on each side.) A single-horned horselike animal has been a symbol of unlimited or undivided sovereign power since pre-Christian times. It appears as a real though "rarely seen" animal in medieval bestiaries, said to come from the Middle East or the Orient and to have the power to neutralize poison. This association with nobility and purity may be the reason the unicorn became associated with the Virgin Mary and, by extension, a symbol for chastity and even monasticism. Today the unicorn has been reduced to a symbol for "cute." Which is also the answer to your second question. Nowadays vampires need ratings. Which means TV appeal. Which means no one over 30, please.