Do this; Achieve that.
Here is a bold statement:
Science, here and now, could turn you into a horse!
I mean that: skeletons can be altered; skin and hair can be grafted; prosthetics can be designed; genetic mapping is only just beginning to illuminate super-healing properties latent within our bodies. You might only be able to become a cosmetic copy and you might have to put up with being a very small horse, but it is possible. BUT!!! there is always a catch, isn't there? Possible doesn't mean feasible; it doesn't mean affordable, and it never means moral. The idea of bodily transformation into an animal is repellant and frightening to most people. This isn't something you are ever going to see on the National Health Service, and probably not even in the most way-out private cosmetic clinics.
Nevertheless, here is how things might be achieved...
|Radical Surgery - using surgery to reshape the body.|
|Nanotechnology - using microscience to reshape at a celular level.|
|Brain Transplant / Clone Technology - build a new body and move in!|
|DNA Manipulation - genetic-level shape-shifting.|
|Virtual Reality Simulations - becoming a horse in cyberspace.|
|Lucid Dreaming - creating custom dreams.|
|Mass Energy Conversion - Star Trek technology.|
|Spontaneous Quantum Tunnelling - possible but infinitely unlikely!|