Researchers from the Norwegian Veterinary Institute developed and tested a system for horses and people to communicate using a symbolic language. From the Daily Grail:
...Twenty three horses learned to tell trainers if they wanted to wear a blanket or not. Subjects were shown three symbols: a horizontal bar to say "I want a blanket", a blank square for "No change", and a vertical bar for "I don't need a blanket". They learned the meanings in a day or two and using them to convey if they were too warm or too cold, building the case for self-awareness...
(In the scientific paper, the researchers write that,) "When horses realized that they were able to communicate with the trainers, i.e. to signal their wishes regarding blanketing, many became very eager in the training or testing situation. Some even tried to attract the attention of the trainers prior to the test sit- uation, by vocalizing and running towards the trainers, and follow their movements. On a number of such occasions the horses were taken out and allowed to make a choice before its regular turn, and signalled that they wanted the blanket to be removed. It turned out that the horses were sweaty underneath the blanket."
"Horses can learn to use symbols to communicate their preferences" (Applied Animal Behaviour Science)