Which of the following is essential for endotracheal intubation in horses?

For endotracheal intubation in horses, using a mouth gag to hold the jaws apart is essential due to the anatomy and behavior of the horse. Horses have large mouths and powerful jaws, and they tend to clench their teeth, making it difficult to access the trachea for intubation without a means to keep the mouth open. The mouth gag allows for sufficient access to visualize the larynx and accurately place the endotracheal tube without interference from the jaws.

While other items on the list, such as a syringe for medication or gauze to secure equipment, may be useful in different contexts, they do not directly facilitate the intubation process itself. A suction device is also helpful in ensuring the airway is clear but is not essential at the moment of performing the intubation. The primary concern during endotracheal intubation is to ensure the clinician has adequate access to the horse's trachea, which is accomplished with a mouth gag.