Does being proficient with a horn count as being "... proficient with wind instruments"?

I'd say so because horns are not string instruments like lutes and lyres, nor are they percussion instruments like drums. 

Horns are Brass instruments.

Wind instruments are Flutes, Oboes, Clarinets, etc.

Quote from starryknight64 >>

Does being proficient with a horn count as being "... proficient with wind instruments"?

 As always, it's up to the DM! Typically horns are brass, but if that keeps some cool story stuff from happening, who cares?

Brass player here: Horns are wind instruments. You blow into them and they make noise. Other commenters are confusing wind instruments with woodwind instruments, a specific type of wind instrument (not to be confused with brasswind instruments), that include bagpipes, birdpipes, (pan) flutes, shawms, songhorns, or thelarrs. Granted, these pipes are very clearly also a woodwind instrument, so if your DM (or you, as the DM) decides they are not playable by a character who has proficiency in the Glaur, Horn, or Longhorn, that would be fair as well.

great for any neutral/evil bards who want a sort of Gothic/ghost motif.

I actually played a skeleton bard that had pipes of haunting.

One of the best items for Fey wanderer. (Talking about the level 11 ability)

Doot season is upon us!

Should this be treated as "a wind instrument"?

Hm is anything immune to frightnened as a general rule? (Undead for instance I wonder if can be frightened)

Is mayonnaise a wind instrument?

How do you get these?

These would be nutty on an oath of conquest paladin

Ah, you mean the 7th. That is indeed badass.

Quite a few things are immune to frightened, yes. Mindless things, undead, and constructs all come up, but there are more.

https://www.dndbeyond.com/monsters?filter-condition-immunity=5

The Pipes of haunting doesn't show up in the character sheet even though it's equipped/"in use" in equipment. Does anoyne else have this problem?

Edit:  Sry, I see now that it is there. Not under Actions/Attack, but in Inventory

This apparently doesn't require concentration, making it an incredibly powerful uncommon magic item for T1/T2 play. (Might be less of an issue with T3/T4 with a lot of creatures with fear/charm immunity or magic resistance/high Wis saves.)

Three times per day is nutty (I homebrewed it down to one for my bard), and DC 15 Wisdom means that the majority of monsters will fail this between 55-65% of the time. A PC with this could, for instance, double-up with Fear or Hypnotic Pattern on back-to-back turns (or, bard/sorcerer could do this with quickened HP/Fear on the same turn, since it's not a spell). Suddenly the entire encounter is either incapacitated or unable to move closer to the party so die by a million arrows and firebolts. Could turn into a magic item you really have to encounter design around for certain PC builds.

So, have fun giving it out, easily reskinnable to a variety of instrument types, but be open to some light homebrewing to make it less encounter-breaking.

Easy nerf for balance would be for it to require concentration to maintain...I might approach my player about that nerf.

Quote from starryknight64 >>

Does being proficient with a horn count as being "... proficient with wind instruments"?

Yes. Of the musical instruments in the PHB, the following are wind instruments: bagpipes, flute, horn, pan flute and shawm.

How much time money and materials are needed to create this?