The story
From Australian rhyming slang dead horse (meaning "tomato sauce" or "ketchup") + -phobia. The slang term dead horse is attested from the late 19th century in Australian and British slang. The pseudo-Latinism mortuus equus (literally "dead horse") was coined humorously to give the term a scientific-sounding name.
Etymology from Wiktionary, CC BY-SA 4.0.