Welcome back to "Fact or Fiction" a/k/a "Quick Answers to Quick Questions" a/k/a QATQQ f/k/a "I don't feel like writing a long blog post".
So, is a miniature horse a reasonable accommodation under the Americans with Disabilities Act? Maybe.
Title I of the ADA covers employers discriminating against qualified individuals with disabilities in terms, conditions, and privileges of employment. There is nothing explicit in Title I or its supporting regulations that suggests that a miniature horse is or is not a reasonable accommodation for an employee. (Miniature horses generally range in height from 24 inches to 34 inches measured to the shoulders and generally weigh between 70 and 100 pounds.) That said, if the request is reasonable (i.e., would not cause the employer undue hardship) and there is no other reasonable alternative, then giddy-up.
The final regulations implementing the ADA for title II (State and local government services) and title III (public accommodations and commercial facilities), which went into effect earlier this year, have a new, separate provision about miniature horses that have been individually trained to do work or perform tasks for people with disabilities.
Entities covered by title II and III of the ADA must modify their policies to permit miniature horses where reasonable, subject to a four-factor test found here.