Book 14, The Unknown, prominently features horse Controllers. From Wikipedia, or equivalently the Animorphs Wikia (the content of both pages appears to be the same):
Cassie and Rachel drive with the former's father, Walter, to the Dry Lands outside of town. 'Crazy Helen', a client of Walter's, has informed him of a sick horse stumbling about the plains. Helen rants on about Martians while Walter and the girls search for the horse in the dark. Rachel spots it trying to make a call at a pay phone. The horse then tries to escape, but is too weak to walk, and falls over. Cassie and Rachel notice a Yeerk falling out of the dying horse's ear. Cassie, getting a bad feeling, tells Rachel to run. A second later, they're knocked over by an explosion. Cassie wakes up in Crazy Helen's caravan, who insists 'it was the Martians!'. Cassie and Rachel decide she is half right - and assume that the Yeerks were trying to keep them away from the horse.
Back in the Dry Lands, the teens morph their respective horse morphs. They join the alleged Yeerk herd, and trot directly into the base. They hear the horse-Controllers speak Galard - the interstellar lingua franca. Ax translates for them, telling them that the Yeerks are planning to complete their mission tonight. The Yeerks promptly break into a run, and rush into a hangar. There, the Animorphs see what the government's been hiding. They know it's not human, but they don't know what it is. Neither do the Yeerks.
According to the Animorphs Wikia's list of Controller species, this appears to be the only time non-human Earthlings have been used as Yeerk hosts.