Loup-Garou is the French name for "werewolf", but in Dungeons & Dragons, it's actually a related but separate breed of monster. Originating from the Ravenloft setting (because what else were you expecting?), loup-garou of the Demiplane of Dread are basically "dire werewolves", being werewolves turned up a level. They come in two forms; the Lowland Loup-Garou, which is more powerful than the standard werewolf, and the Mountain Loup-Garou, which is more powerful than its lowland cousin.
Both iterations of the loup-garou are bigger, stronger and much tougher than the standard werewolf. Lowland loup-garou can assume the form of a worg or a humanoid worg in addition to their base human form, and are much more resistant to magic and even magical weaponry; whilst a silver weapon still does full damage, a magic weapon only does damage equal to its bonus - so a +5 Sword will only do five damage to a lowland loup-garou on a successful hit. Given that lowland loup-garou are almost identical to regular werewolves apart from their larger stature, this can be a very fatal surprise to an unprepared warrior. That said, being touched by the Dark Powers means they take a bonus +1 damage from a Blessed weapon, providing it's already magical or silvered. Mountain loup-garou are literally just bigger and nastier; they have a Dire Wolf alternate form and hybrid form, they can Regenerate (albeit only 1 HP per round in AD&D), and just for added kicks, they're vulnerable to gold, not silver.
In 3rd edition, these abilities were downplayed; the lowland has Spell Resistance 12, whilst the mountain has Spell Resistance 14 and Fast Healing 1.
Apart from that, there's really not much to say about the Ravenloftian loup-garou. They're basically just werewolves, but with nastier statblocks.
These lycanthropes debuted in the Realm of Terror boxed set for Advanced Dungeons & Dragons, and were then reprinted in the Ravenloft Monstrous Compendium Appendix III. They were subsequently updated to Dungeons & Dragons 3rd Edition in the Denizens of Darkness/Dread duology.
They returned to Dungeons & Dragons 5th Edition in Van Richten's Guide to Ravenloft. Here, in addition to a buffed stat block, what makes them so nasty is that those werewolves created by their bite are incurable so long as the loup-garou lives.