Please refer to the 'Getting There' section on the main page for driving directions to the Horn Creek Trailhead.
RT Distance: 9 miles
Elevation Gain: 4500'
From the Horn Creek TH, hike past the outhouse and the sign indicating Rainbow Trail -- .25 miles (BTW, it's more than .25 miles away, so don't sweat it if you think it's taking too long for a quarter-mile). You'll reach a 4-way junction with another trail shortly. Don't go straight, despite the lack of signage, but turn right, and you'll be on the Horn Creek Spur Trail, which will take you the rest of the way to Rainbow Trail (heading straight also takes you to the Rainbow Trail, but you'll be .5 miles south of the Rainbow Trail's junction with the Horn Creek Spur Trail.... this *is* however the way you'd want to go if you're headed for the Horn Creek Trail or the Macey Lakes Trail).
At the Rainbow Trail, turn right. After a brief walk, you'll pass the Dry Creek Trail and you'll cross Dry Creek itself shortly after that. You then have approximately .75 miles to the junction with the Horn Peak Trail. At the junction, take a moment to sign in at the wilderness register and then start heading steeply uphill. After climbing ~1000 feet, you'll reach a crossing of Hennequin Creek.
After the crossing, the trail switchbacks and climbs, reaching a ridgeline at ~10,700'. The trail turns to head SW up the ridge, leading eventually to UR 12,665 on the Horn Peak quadrangle (the trail itself is not on the quad, which hasn't been updated since 1959). From there the hike is a class 2 talus climb to Horn Peak's summit, which features spectacular views especially of the Crestones, Mount Adams, Kit Carson Mountain, Fluted Peak, Little Horn Peak, Colony Baldy, and Humboldt Peak.
On the descent, take care not to get off-trail and follow a steep, loose trail down a gully that appears to have been the old route up Horn Peak from Hennequin Creek. As of 2003, the FS is in the midst of restoration efforts on this dilapidated old trail.
Typical hiking gear, nothing specialized required unless conditions dictate.