Name: Mount Call. P/W/C Type: Courage. Appearance: A large charm in the shape of a horse’s iron. At the top, there’s a mouthpiece. Description of Function: Functions as Epona’s Song, except much faster. Limitations: None that I can think of. Same limitations as Epona’s Song? Description Suggestion (Move your mouse to reveal the content) Description Suggestion (open) Description Suggestion (close) `Mount Call Canon – Magic-tier, Utility. [travel] [pricing] This horse iron-shaped charm has a mouthpiece at the top, being a cleverly-disguised whistle. When blown, the item magically produces a short melody which will summon (one of) the user’s mounts. This cannot bring a Mount across different dimensions however, such as bringing a horse from Hyrule into the Dark World.
The only problem is that the original Epona's Song doesn't say anything about the time required to cast. Having the Mount Call, then, wouldn't really let a character do anything new. Its use in-game was to allow the player to summon Epona regardless of position (without those reeds), but that's already doable here, using the OoT Epona's Song. Perhaps if the Call had an additional affect or something. As it is, this does almost exactly the same thing.
Some characters with mounts are NOT musically inclined, though. I have several characters with mounts or plans for mounts that singing is just either out of character or just terrifying for. Hence the Mount Call- a way to summon your mount without first making a total fool out of yourself.
Sounds like an instrument to me, even if it is capable of only playing one song. Epona's Song doesn't say how the tune needs to be played, only that the tune needs to be played. You could get away with this on your own, if your character isn't musically inclined. Personally, I don't think you'd need to make a treasure for that.