Hi Audrey, those are all good questions and I will give you my honest opinion on all of them. As for riding horses, it's not something that I ever really did so I can't say for certain. A couple years back I went to Cuba and I went on a horseback riding tour and I was able to do it although the horses were just walking. If the horse was to start running it might be a tad difficult but I have no experience so you may well be able to develop a technique that works. I will say it did feel a bit awkward not being able to bend my one leg but I think it could be fixed if the one stirrup was longer on the saddle. Could you get hurt due to your leg being fused? Unfortunately in our world Audrey that is always a possibility. I have been hurt many times because of my leg.
Based on what you have said it's clear you still have lots of pain which will take time to go away. This will also hinder some of the things I will tell you but it will come with time I gaurantee it. Going up stairs I usually take two stairs with my good leg and then one with the bad and repeat. I developed this more to not be slow and hinder people behind me. My fiancee gets mad at me because I'm always trying to be considerate for others and she feels it should be the other way around. But, bare in mind my good leg is much stronger because it is basically doing two legs worth of work. Especially, over 20 years it has had time to get strong. Going down stairs, I do a kind of hip hop step which I also developed to be speedy. I will try and put some video clips together to show you better. Josephine mentioned you can upload you tube links so maybe I will try that.
Ah yes, the trusty bicycle. As a kid I loved riding my bike, it was my favourite thing to do. I remember being in the doctor's office before it got fused and I asked the doctor if I could still ride my bike. My doctor told me I would never ride a bike again. I was crushed!

My knee was fused weeks later and my leg was in a cast for four months. When they took off the cast and I was off crutches and could somewhat walk again the first thing I did was head to the garage and grab my bike. I remember saying to myslef, "I'm going to ride this thing!" I developed the right heel tap which did the job. I will say this quickly as I will touch on it after with the quad muscle question but your hip and ankle are going to become very strong and play a huge role in your life going forward. My right hip is very strong. Your hip will be a major part of riding a bike if you choose to. First, you need to be able to hold your leg up in the air which will make your hip sore and tired in the beginning until you get stronger. After that it's just basically a case of as the right pedal reaches the bottom of its rotation I hit it with the back of my shoe to help it go back up. It's looks funny and I got stared at like crazy but I didn't care I was riding my bike!

Once again video would do it more justice. I even have a technique for going up hills too.
My knee has absolutely no bending. It is straight as a board. I believe when they fused my knee they actually cut the tops off of my bones and let my femur and fibula/tibula grow together. My right leg is essentially one bone now I believe. Have I ever bent it after the fact? Yes, a few times and it sucked big time each time it happened. It hurts like hell and I would recommend not doing it but sometimes things can't be avoided. Most were sports related which means they could have been avoided if I wasn't playing sports but that wouldn't be my style. My mom developed lots of grey hair. I did however have a bad car accident in Portugal in 1991 and did some serious damage to it. I broke the fuse along with damaging the growth plates in my bones so my right leg stopped growing like my left. This resulted in me needing leg lengthening surgery (ilizarov surgery) and orthotics which I still wear as my right leg is still a bit shorter.
As I mentioned above, your hip and ankle will become very strong and also become very important in your mobile life going forward. I can't tell you how many times I have rolled my ankle because I can't bend my right knee. It doesn't take much either. Due to the fusion, my right leg is considerably smaller than my left. My right foot is one shoe size smaller because I tend to stand on my good leg. I basically have a small quad and hamstring. it's hard to work them when you can't bend your leg. Leg raises anyone!!!!!! My right butt cheek is even smaller than the left. However, my lower leg (calf) looks exactly the same as my left.
I'm really starting to see that maybe tkr isn't for me. I have done so much in my life with my knee and leg they way they are. I don't know if I got it done and I encountered problems like you I could handle it. To go backwards would be a huge blow to me and what I have accomplished. But, you must always look at all the options and go from there. Let's see what the future holds. I'm always a type away so please feel free to keep asking me things. I will try and get those video clips as seeing is believing. Good luck Audrey and keep staying positive! Pain heals!