Human Pack Mule

When a character is shown to be the personal carrier on which all other members of the traveling party dump their belongings onto, much to that person's chagrin and everyone else's amusement. The person in question usually does not possess Super Strength (or even above-average) Strength, so the experience is usually an unenjoyable one for them.

Seen often in episodes where the women take a man shopping with them—the man almost always carries the shopping bags.

However, a subversion is occasionally seen, in which the guy is all too happy to demonstrate his strength to onlookers.

Examples of Human Pack Mule include:

Anime and Manga

Kyon of Haruhi Suzumiya is almost always carrying everyone else's stuff, though occasionally Koizumi lends a hand.
Dragon Ball: Goku usually takes this role in the Filler episodes whenever Chi-Chi goes shopping. Being a Saiyan with a ridiculous power level, this means Goku's usually lost behind an epic mountain of shopping bags.
Kenichi the Mightiest Disciple: Kenichi is this to Miu whenever she goes shopping for groceries. This, of course, doubles as a part of his Training from Hell, and his mentors often force him to carry some extra weights on those occasions, just for the sake of training his strength and endurance.
In the beginning of Rance Sabaku no Guardian (the Hentai Anime of the Game for Rance), you get the first hint that the Designated Hero is a sexist pig by him having his slave Sill carry a huge load of useless stuff.

Comic Strips

FoxTrot: A trip to the mall with Paige (theoretically to buy a pair of shoes) ends in Peter carrying a dozen bags, none of which contain shoes.

Films -- Animation

In Little Nemo Adventures in Slumberland, Flip is assigned this role after his previous job as a guide is taken by some friendly locals.

Films -- Live-Action

Spaceballs: In one scene, Lone Star and Barf end up carrying all of Princess Vespa's luggage, including a giant hairdryer.
Patsy, the coconut guy from Monty Python and the Holy Grail.
In one film of the Danish Olsen-Banden series, Egon Olsen, the head of the group, had to carry the computer hardware (in a few very large suitcases) for the new, modern head of the group.

Literature

The Lord of the Rings: Samwise Gamgee inflicts this on himself, being stalwart and all.
And in The Hobbit, it's a running gag that Dori always ends up carrying Bilbo.
Fezzik in The Princess Bride is this for both the bad guy and good guy teams he ends up on. However, he is a Gentle Giant with particularly strong arms, so he doesn't mind too much—except when he has to haul the entire party up the Cliffs of Insanity.

Live-Action TV

Used in Buffy the Vampire Slayer with Xander.

Music

The music video for "Gonna Take a Lot of River" by The Oak Ridge Boys (forward to 0:35).

Tabletop Games

This is a general occurrence in these types of games, sometimes with characters being carried as well.

Theater

It's written into the stage directions of the Opening Ballet of Guys and Dolls.

Video Games

As the only player character in Resident Evil Outbreak with more than four inventory slots, Yoko is often relegated to this role. A popular strategy for online gamers was to kill the Yoko player and use her corpse to store extra items.
This seems to Yun-seong's role in the Soul Series, courtesy of his Cool Big Sis Seong Mi-na (as seen in Mi-na's ending in Soulcalibur II and III, as well as Yun-seong's B ending in III). However, if the player does not press the right button(s) in time during Seong Mi-na's ending in III, Mi-na herself will have this fate befall her.
EverQuest players are known to create characters just to hold overflow items and money.
Lampshaded by several of the companions in Fallout: New Vegas. Raoul, having lost faith in his abilities suggests you assign him this role. Veronica sometimes complains that you're making her carry "the heavy stuff" when you access her inventory and Arcade Gannon specifically warns you not to treat him as a pack mule when you open his.
In Metal Slug, supply sergeant Rumi Aikawa is seen carrying an oversized backpack holding an incredible amount of cargo. She appears in several locations, trying to deliver these weapons and items to friendly forces (i.e. the player). Her older (but identical-looking) sister Madoka takes up this role in 4 and 5.
The role of followers in The Elder Scrolls V: Skyrim amounts to this for many people, as well as helpful back-up and (depending on the character) interesting comments.

Web Comics

The entire job of henchmen in Nodwick.
In Drowtales, Liriel assigns Rik this role.

Western Animation

An Imagine Spot on The Simpsons has Lisa & Maggie along with their hypothetical little sister, now adults, out shopping, with Bart along as their package carrier.
The Fairly OddParents
- While not done properly, one episode of used this as a gag. Timmy even was fed with a... thingy like the one used on horses.

Timmy: And I'm sick of eating these oats... even if they are low in fat.

- Played straight in "Timmy the Barbarian", a Conan the Barbarian parody told by Jorgen Von Strangle to Binky as a bedtime story. Cosmo was not only the "no respect" carrier of the group's bags, but Jorgen's narration turned him into a literal pack mule just to add insult to injury.
Roger Rabbit, in the theatrical cartoon Trail Mix-Up.

Real Life

For a long time it was common for the larger exploratory expeditions to carry the "stuff" (including rations, weapons, camping equipment, scientific equipment, and all sorts of things). Often they did it on the backs of humans rather than animals because the terrain was so bad that a human was the best choice.
- Sherpas are famous as Badass Human Pack Mules.