They are one of the quest items for a Heartwood quest which ends with a final reward of a Strongbox, which is the best quest loot you can get. It starts with Lorekeeper Broolol and a map fragment which is obtained from the red NPC healers, the Priests of Mondain. You can find more information on the quest here:
Knowledge of the Legendary Minotaur
Along with 1 Fragment of a Map and 20 Gaman Horns, you will also have to acquire 3 Minotaur Artifacts per quest. The Minotaur Artifacts only spawn in the Labyrinth in the area that is past Miasma, then past where Rend spawns, then even farther past where Pyre spawns. Best way to obtain these is to take a character with Magery, both Hiding and Stealth, and an empty pack. You can stealth past everything fairly easily except for Pyre, who has an area effect Fire which will reveal you. Also, the Minotaur Scouts have an increased ability to reveal stealthers. The Minotaur Artifacts are Blessed, so if you die in the process of trying to get out after grabbing some, just go outside the Labyrinth to the Healer, get rezzed, and they will still be in your pack. If you get revealed while you're inside, simply cast invis (helps to have FC/FCR jewels). There are no magic casting creatures in there, so it's a simple matter of invising before they can reach you. The only ranged attack creatures are the Reptalons, but they are very slow, so not really much danger at all.
I usually grab around 15 of the artifacts (that's about the total number that spawn, so it's one trip in to grab all that are there), 5 of the Fragment of a Map, and 100 Gaman Horns, which will allow you to quickly complete 5 quests, and so obtain 5 Strongbox rewards. It's well worth the time and effort as properties on the items contained in the boxes are at 80% and higher, so items that have the "good" properties all on a single item are Peerless/Dark Father quality items.
