There are already 2 known dire cows: Apocalypse Cow (from the tv series, The Tick...formerly the normal-sized, homocidal bovine, The Man-Eating Cow, from the comic and cartoon) and Babe, Paul Bunyon's blue Ox.
You could really screw with the PCs with a dire cow. Tell them they need an object from some villagers. These people will gladly give it to the PCs if the party will go fetch milk from their sacred cow, which has been kidnapped.
Imagine the looks on their faces when, after fighting their way through monsters and traps, they throw open the door and discover the cow in question is 30 ft tall.
Now they have to milk the damn thing without hurting it (it is sacred after all). Give the PCs a situation they have to think through without pulling out their swords.
Or you could really screw with them and have it suffering from Mad Dire Cow Disease.
But a better question is "What would a half-fiend dire cow do?". That would freak out the players. Fire and milk shooting from the sky. Beware the beat of bovine wings!
Serious though, a dire bull
would be pretty cool (like Taurus, the bull killed by Hercules).