The Mul is a race of human/dwarf crossbreeds within Dungeons & Dragons that originated in the Dark Sun campaign setting, wherein dwarves are primarily a slave race. The idea was that forcing humans and dwarves to breed would produce a race that would hopefully have human flexibility and stature, combined with dwarven strength and endurance, thus producing a superior slave-race. The result worked more or less like a charm; muls are taller than normal humans and massively muscled, with absolutely incredible physical prowess. They are, however, usually completely hairless, sporting at most some hair on their head -- but then, Athasian dwarves were usually depicted as being completely bald as well, with Klingon-style forehead ridges to boot, so that's not quite so weird.
Dark Sun being the grimdark D&D setting of its time, muls were stated as being very prone to dead mother syndrome - since a baby mul is so much bigger than a human or a dwarf baby, birthing is very difficult - and also to be sterile, despite the fact that half-elves and half-orcs have never been described that way. The idea was to emphasize just how cruel and callous the Sorcerer-Kings are, in that they would force the creation of muls even knowing they were costing women their lives for something that would live one generation and then be gone, all so they could have super-good slaves and/or gladiators, and reinforce the idea of them as a bred-for-slavery race, strengthening the otherwise name-only comparison to real-life Mules.
4th edition, being the lighter & softer edition in many ways, abandoned both of these aspects, and generally made them (and the entire setting of Dark Sun) more mainstream-appealing and less original and grimderp.
In fact, 4th edition actually devoted an article in Dragon Magazine #391 to examining a more setting-neutral possible origin for muls, in this case tying them to a drow city-state that had a Lolth-inspired vision to breed super-slaves from their human and dwarven slave-stock, only to lose control of them when the fickle bitch lost interest. It was suggested that, like in the original setting, these muls were probably very staunchly anti-slavery and had a strong anti-authoritarian streak. In difference to the original setting, however, they were said to not get on well with dwarves, due to many dwarves seeing their half-human origins as being abominable.
In the original AD&D setting (and in the fan-made, but WotC-approved, update for 3.X), their general outlook on life was, as stated, similar to the Dragon Magazine suggestion, and they were a fairly strong racial choice for martial classes - well, barring the (in the right build, quite OP) Half-Giant. They were also, from a player/real-life human-perspective, quite a bit "uglier", but being made up of mostly musclegirls on the female end of things.
Incidentally, their name has long been a source of some minor debate; why are they called Muls instead of "half-dwarves"? And how do you pronounce it? In 2e, it was generally accepted it was pronounced "mule" and was given as an insult (and arguably an analogue for the real-life word "Nigger"), another way of highlighting their "born to slavery because of bastard overseers" origin. In 4e, as part of "softening up" the setting, it was changed to being short for Mulzhennedar, the dwarven word for "Strength", and it was noted that the two most common acceptable pronunciations were "Mull" (probably the "purest" pronunciation) and "Mool", with "Mule" being a good way to start a fight.
In Advanced Dungeons & Dragons, muls had minimum Strength scores of 10 and Constitution scores of 8 (bumped up to min Con 10 in the Revised version) and maximums of 20 , paired with +2 Str, +1 Con, -1 Int and -2 Cha as racial modifiers. They also had a really weird focus for racially specific abilities, in the form of... Work. No really, due to their mix of human and dwarf parentage that was one of their special gifts. The ability to work for literally days at a time before needing to stop and rest. Generally speaking, this had zero practical use campaign-wise as there aren't many GMs who are going to feature a campaign where you have to dig a ditch in a hurry.
They make good tanks and in particular they made excellent choices for Gladiators, otherwise known as "Fighters better than all other fighters." The damage a Mul gladiator with a strength of 20 could inflict with Cahulaks (a type of weaponized grappling hooks tied together by rope) was a beautiful thing to behold.
In 3rd edition, they appeared in Dragon Magazine #319, where they had +2 Strength, +4 Constitution, the "Hardy" racial trait (requires only half the normal amount of sleep to eliminate fatigue & exhaustion), the Faster Natural Healing racial trait (recover 2 hitpoints per day, gain Fast Healing 1 at 10th level, increase to Fast Healing 3 at 15th level), and an Inborn Power trait that gave them 3 psionic power points and the innate ability to use Offensive Prescience with them, which just got added to their class-based psionic power pool and powers if they took one of the psionic classes. They had a Favored Class of Fighter (although you could argue that Psychic Warrior was just as fitting) and a Level Adjustment of +1.
For the 3.5 rules revision, Mul Racial Traits were semi-officially published by Athas.org, whose conversion of all 2e Dark Sun materials were considered "officially approved" by WotC. In Dark Sun 3 (revision 7), the stats given deviate heavily from the stats given by Dragon Magazine for 3e. Mul gain +4 Strength, +2 Constitution, -2 Charisma. They count as Medium-sized Humanoids with the Dwarf subtype, and have Darkvision. They have regained the 2e bonuses to hard work with "Tireless", granting a +4 racial bonus to checks for performing physical activity over an extended period of time and +4 saving throws against spells that cause any form of weakness (fatigue, exhaustion, enfeeblement, etc.), as well as "Extended Activity", allowing them to engage in up to 12 hours of hard labour without becoming fatigued. They also count as a dwarf for all effects related to race and have a Damage Resistance of 1 vs. Nonlethal damage. Their favored class is Gladiator, their automatic language is common, and they have a level-adjustment of +1.
4th edition, being somewhat more mainstream, instead gave them a static race bonus of +2 Con and then their choice of either +2 Str or +2 Wis, a bonus healing surge, the ability to count 6 hours of sleep out of every 72 as a long rest, and the ability to automatically end any ongoing damage/dazed/slowed/stunned/weakened effect once per encounter due to being so goddamned tough they could just shrug it off.
|Dungeons & Dragons 2nd Edition Races|
|Core:||Dwarf - Elf - Gnome - Half-Elf - Half-Orc - Halfling - Human|
|Dark Sun:||Aarakocra - Half-Giant - Mul - Pterran - Thri-kreen|
|Dragonlance:||Draconian - Irda - Kender - Minotaur|
|Mystara:|| Aranea - Ee'ar - Enduk - Lizardfolk (Cayma - Gurrash - Shazak) |
Lupin - Manscorpion - Phanaton - Rakasta - Tortle - Wallara
|Oriental Adventures:||Korobokuru - Hengeyokai - Spirit Folk|
|Planescape:||Aasimar - Bariaur - Genasi - Githyanki - Githzerai - Modron - Tiefling|
|Spelljammer:||Dracon - Giff - Grommam - Hadozee - Hurwaeti - Rastipede - Scro - Xixchil|
|Ravenloft:||Broken One - Flesh Golem - Half-Vistani - Therianthrope|
Book of X:
| Alaghi - Beastman - Bugbear - Bullywug - Centaur - Duergar |
Fremlin - Firbolg - Flind - Gnoll - Goblin - Half-Ogre - Hobgoblin
Kobold - Mongrelfolk - Ogre - Ogre Mage - Orc - Pixie
Satyr - Saurial - Svirfneblin - Swanmay - Voadkyn - Wemic
|Dragon Magazine:||Half-Dryad - Half-Satyr - Uldra - Xvart|
|Dungeons & Dragons 4th Edition Races|
|Player's Handbook 1:|| Dragonborn - Dwarf - Eladrin - Elf |
Half-Elf - Halfling - Human - Tiefling
|Player's Handbook 2:||Deva - Gnome - Goliath - Half-Orc - Shifter|
|Player's Handbook 3:||Githzerai - Minotaur - Shardmind - Wilden|
|Monster Manual 1:|| Bugbear - Doppelganger - Githyanki |
Goblin - Hobgoblin - Kobold - Orc
|Monster Manual 2:||Bullywug - Duergar - Kenku|
|Dragon Magazine:||Gnoll - Shadar-kai|
|Heroes of Shadow:||Revenant - Shade - Vryloka|
|Heroes of the Feywild||Hamadryad - Pixie - Satyr|
|Eberron's Player's Guide:||Changeling - Kalashtar - Warforged|
|The Manual of the Planes:||Bladeling|
|Dark Sun Campaign Setting:||Mul - Thri-kreen|
|Forgotten Realms Player's Guide:||Drow - Genasi|