It depends on the severity of the injury.
Desitin is zinc-oxide based cream. It acts as mild astringent and has weak antiseptic properties. It is used to treat and prevent various skin conditions including minor abrasions, burns, chafing, insect bites, and minor skin irritation.
If the horse has a minor abrasion or a slight scrape, then yes, using Desitin might be helpful. However, if the injury is more severe, then using an antibiotic ointment is the best option.