Channeling can involve any energy. You can, for instance, channel the energy of water to heal or channel the energy of an animal to take on their personality traits and mental characteristics. You can also channel deities in this way, but it is not necessarily intelligent. It ends up being interpreted and expressed through you in an individual way based on your chemistry with it.
Horsing is specific to channeling a fully conscious intelligent being with your body as a vessel.
Here is an example. If you invoke a rune, you are channeling it's essence through yourself and using your will and concentration to focus it's power to manifest your goal. You are the catalyst.
If you invoke a spirit, you are channeling a part of its "mind soul" or what you may consider its consciousness through yourself. This is horsing. You are not in control of what it does. You are handing your reigns to it so that it can use your body. It is independent. You can only limit what it can and cannot do through you. It is approached as an individual and must be treated as such. This is why horsing is dangerous and should never be attempted alone.