I think part of the reaction here is the very idea of a horse. A horse! On a plane.
To read your link though, it says "reasonable" accommodation. It specifies that part of the assessment of reasonable accommodation is if the horse must be house broken.
Can you imagine having a non-housebroken miniature horse in your row with you? I find this hilarious and I hope it happens to me just for the story.
(i)Miniature horses.
(1)Reasonable modifications. A public entity shall make reasonable modifications in policies, practices, or procedures to permit the use of a miniature horse by an individual with a disability if the miniature horse has been individually trained to do work or perform tasks for the benefit of the individual with a disability.
(2)Assessment factors. In determining whether reasonable modifications in policies, practices, or procedures can be made to allow a miniature horse into a specific facility, a public entity shall consider -
(i) The type, size, and weight of the miniature horse and whether the facility can accommodate these features;
(ii) Whether the handler has sufficient control of the miniature horse;
(iii) Whether the miniature horse is housebroken; and
(iv) Whether the miniature horse's presence in a specific facility compromises legitimate safety requirements that are necessary for safe operation.
(3)Other requirements. Paragraphs 35.136(c) through (h) of this section, which apply to service animals, shall also apply to miniature horses.