Research suggests horse yawning is linked to social stress, testosterone, and welfare.
A European study published in "The Science of Nature" set out to explain why horses yawn. The research detailed the behavior's frequency and what a horse's yawn can could signify to its caretakers.
A total of 19 wild Przewalski horses were left with little to no human interaction in a 280-hectare region in France. Additionally, 16 domestic horses of various breeds were observed in outdoor conditions.
While researchers found no difference between the wild and domestic horses, they did find that the Przewalski horses socialized at a higher level than the domestic group. Aggressive Przewalski males also tended to yawn more, in accordance with previous studies on yawning mammals that linked the behavior to changes in social situations such as excitement or stress.
Domestic horses also yawned more frequently when engaging in affiliative behavior, such as sidling up to another horse to increase association with the animal. This also supports the theory that animals yawn due to social behavior.
Further, the study found a correlation between yawning frequency and gender. Stallions in both subject groups yawned more than females, but geldings did not. This relates to the hypothesis that yawning occurs in intense situations possibly to decrease stress.
The researchers concluded that domestication alone does not cause yawning in horses. However, horses kept in more confined enclosures tended to yawn more, indicating the living conditions were somewhat stressful. Horses stabled in more open areas did not yawn as frequently. Therefore, yawning can be an indication of equine welfare in domestic species.
The study also suggests that yawning may be prompted by higher amounts of testosterone in addition to social stress.
So if your horse is yawning, it could mean a variety of things. Not just that he is bored with whatever you are doing.