Dire Wolf
The Dire Wolf is a concept that appears in a lot of fantasy works, based on a simple train of logic: wolves are scary, cool and badass as they are, but they're actually pretty small. How can we make them even more scary, cool and badass? Why, we make them bigger, of course! Whilst there can sometimes be other traits, such as being malevolently intelligent or possessed of magical abilities, at their core, dire wolves are regular wolves scaled up several notches. The exact stature varies, but around the size that you can ride them like a horse is pretty common.
The term is also often co-opted as a badass title for spaceships, mecha, tanks or other mighty war machines, simply because it sounds badass.
In D&D and Pathfinder
The dire wolf is the oldest and most recognizable of the Dire Animals in Dungeons & Dragons and spin-off games like Pathfinder. In fact, there have traditionally been a variety of lupine monsters built around the dire wolf chassis, such as Worgs (evil, sapient dire wolves with a tendency to work alongside goblinoids) and Winter Wolves, who have icy [[elemental]T] abilities.
In Warhammer Fantasy
In Warhammer Fantasy, the wolves of Sylvania are known and feared as some of the largest and most vicious in the Empire. However, the term "dire wolf" is used for zombie Sylvanian wolves used as fast-moving hunters by the Vampire Counts. For more details, see Dire Wolf (Warhammer Fantasy).
In BattleTech
In the universe of BattleTech, the Dire Wolf is an Assault OmniMech invented and deployed by The Clans, and generally regarded one of the most terrifying Mechs ever fielded. See Dire Wolf (BattleTech).
In Warhammer 40K
The Adeptus Titanicus fields a model called the Dire Wolf Heavy Scout Titan. It is generally considered to be a blatant attempt by Warhammer 40,000 to rip-off the BattleTech Dire Wolf OmniMech.