When I played Old D&D with Gygax a few years back he didn't encourage purchasing a mule; instead, he recommended we buy a dungeon cart, basically an adventurer-pushed wheelbarrow to carry treasure. Even after a gelatinous cube ate my mule, my accursed companions put my unconscious elf in the cart and used me to batter open doors. I'm still bitter. My mule barely lasted half an hour.
In comparison, 4e boasts several versions of the mule: the kickgroin donkeyhorse, the cart-rammer donkeyhorse, the gnomechewer donkeyhorse, and the universally disliked bootpiddle plopstep donkeyhorse. I think they're just too powerful for an average group! These things can fight, run, set off traps, act as a mount, act as cover, be used as an alarm, and double as iron rations. Broken.