Hi
@Dakota731 
. Each one of your examples has a slightly different answer and I'll take a stab at them.
First, bikes rented at Disney's Bike Barn between the Campfire/Movie Ring and the Meadow Pool aren't reserved - they are for walk-up DAILY rentals only. Rates a few years ago were $20/day/bike but be aware the bikes CANNOT be kept out overnight. They must be returned to the BB each evening before it closes (so knowing the opening/closing hours is key here). Which means they're good for a one-day experience. If you want bikes for multiple days, a bike shop in Celebration rents and delivers/picks up at the Fort and one of our DIS folks used them about a year ago and was pleased. We always bring our bikes from home (we drive in from Alabama) and the Fort is a great place for biking.
Second, boats can no longer be rented at the Fort marina (and a few others were closed down). When covid hit, ALL boat rentals in WDW were suspended - when covid restrictions were lifted, some resorts did not resume boat rentals. Speculation was it was also time for a vendor contract renewal. There are no more little water sprites anywhere but a few WDW resorts still now rent the pontoon boats (up to 10 people) for $49 per half hour. Nearest locations to the Fort are the Contemporary or Grand Floridian. See this:
https://disneyworld.disney.go.com/recreation/motorized-boats/
Note the link says pontoons are first come, first served (no reservations). We did it once and enjoyed it and likely will again.
Third, by 'horses' I assume you mean the Trail Ride at the front of FW behind the checkin area and NOT the pony rides down at the Tri-Circle-D Ranch near Bay Lake.
Info for the Trail Ride is here:
https://disneyworld.disney.go.com/recreation/campsites-at-fort-wilderness-resort/horse-trail-rides/
Note it has a 60-day booking window like dining reservations do and there are multiple rides per day. Obviously book as soon as you know your vacation schedule.
I hope these leads help.
Bama Ed