Answer: In preface to my answer to your question, please note that the name "red maple" is not necessarily associated with leaf color and that there are many types of maples identified most precisely by their Latin name rather than their "common" name such as "red maple". These differences are very important in terms of your question and the safety of your horses.
According to the veterinary toxicity information available through Purdue University, ( http://vet.purdue.edu/depts/addl/toxic/plant50.htm ) and several other authorities, red maple (Acer rubrum) is in fact potentially lethal to horses. The toxin, however, is not specifically identified either at that data base or in several others I looked at in reference to your question.
Despite the name, "Crimson King' is not technically a red maple or Acer rubrum variety. Instead, this is technically a named variety of Acer platanoides or Norway Maple. I did not find reference to Norway Maple as a cause of toxicity to horses. The databases however are not represented as being complete, perhaps because research continues to be done all the time. It is also possible that no cases of Norway Maple toxicity have been reported in the literature consulted although they may have occurred nonetheless.
Although I have not found any references to toxicity of Norway Maples to horses in my brief research, I strongly suggest you double check this information with your veterinarian. Better yet, you might ask your veterinarian if there are any trees that are actually known to be safe and recommended for planting in or around a horse paddock.
I am sorry I can't be more specific but I think you would agree it is important to be safe rather than sorry.
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