The base peg of the horn/drill-bit would be too wide for the hole, but based off the cast fault line, which I hi-lighted in blue, there is a greater chance for the plastic halves to ease the slimmer screw driver in versus the blunter horn/drill-bit peg. I don't have this Pegasus on hand, but... it just might work here.
This Pegasus first appeared last year and is in only two sets. I'm thinking the LEGO group just hasn't had the proper time to release a FRIENDS-line compatible accessory.
I see three choices:
- We can gingerly test an illegal connection.
- We can perform a Blasphemer's Lobotomy.
- We can wait on LEGO to provide a piece.
EDIT: While we wait for option three, let's play with what we got...
If the problem is that the base of the horn is too big to fit in the hole on the chanfron you have to make the former smaller or the latter bigger. Careful work with a file/scalpel, if you're good at that kind of thing. Remember that it's a lot easier to take a bit more off than to put a bit back.
Odd that the bow doesn't use a standard size stud anyway (walks away mumbling something about the old days).
If you're not afraid to modify (cut and/or bend) one of the horse helmets, they might just fit.
P.S.: If it has wings it's not a horse but a pegasus. (I know, I'm nitpicking here)
It doesn't look like there's currently any way to make this connection by normal means.
While there are quite a few parts with similar 1.6 mm attachment holes as your daughter's horse has (including some mildly surprising ones like this ladybug), parts with the matching pins seem to be relatively few. Basically all of them seem to be small ornaments like minifigure plumes and other headgear accessories, including some Friends accessories like your bow. And as far as I can tell, none of them are really suited for attaching further parts.
Screwdrivers, as suggested by Rin Rio-Oki, probably won't fit: the shaft is slightly too thick, as you can see in this composite photo. The tips of flower stems might fit, but even if that worked, it'd just look totally ridiculous.
So you'll probably have to resort to some "blasphemous" methods. In that respect, I see two options worth considering:
Filing the base of your unicorn horn down to about half of its original diameter, as suggested by BlokeDownThePub, should let it fit into the hole. It'll be tedious and fiddly work, and you could easily end up ruining the part entirely if you're not careful, but with practice it should be doable. Of course, this also means that you won't be able to use the horn for its original purpose any more — although there are plenty of parts that should be able to make the connection in that direction, like the Round 1x1 with Bar and Pin Holder.
You could glue the horn to a part that already has a suitable pin, as you yourself suggest. While you could use the bow you already have for this purpose, I'd suggest trying a Star with Pin accessory instead:
If the 3D models are anywhere near accurate, the base of the horn ought to just fit inside the star (although not all of it; it's too long). Alas, it probably won't stay in without glue, but at least it may almost look like a legitimate connection.