Gary the Horse is a character who has been occasionally mentioned in the various Tackleford strips, but first appeared in person in the Big Hiatus storyline of 2016, when he brought Shauna to the place where various people including some of the other Mystery Kids had been turned into statues by the Green Man. Unfortunately, after taking her there, Gary proceeded to dunk Shauna in the water, because he said he was a Brag and it was in the nature of Brags to do this. Shauna then told him it was going on his Wikipedia page; the Tackleford Wikia will have to do - and likewise the Wikipedia page on Brags. (However, that entire storyline is only given as being told by Lottie to Shelley, as an unreliable narrator, so this incident may not have actually happened this way or at all.)
In earlier mentions, Gary was in news articles for a crime spree in 1960, where he committed jewel robberies, was captured by the police, and later escaped and caused the deaths of Ron and Nellie Goater (parents of Calvin, who appeared in the Case of the Forked Road) in a car crash. Gary is apparently either very long-lived as horses go, or his name has passed to several horses in different generations.
He appears to currently be a resident of the fairy glen which moved to a pocket universe after its location within Tackleford became the site of a housing development.
Gary does also appear in this piece of promotional art, together with Bobbins era Ryan, Shelley and Amy.
A fictionalized analogue of Gary the Horse was alluded to in CONAN & THE BLOOD EGG (1 of 3), implied to be the unseen mount of Shailey (fantasy Shelley Winters).