Cavalier

The Cavalier, based on the long and romanticised history of the horse-riding, lance-wielding knight, is one of the bastard stepchildren classes of Dungeons & Dragons, something that almost everyone tries to sweep under the rug, yet which keeps popping back into the light no matter what is done by the fans.

There are two specific breeds of Cavalier in the context of D&D. The original variety, known today as the Cavalier-Paladin, is a horror that only neckbeards of Old School Roleplaying lineage have experienced, and which has been more or less successfully stamped out, and the "proper" cavalier, who has manifested itself once per edition ever since.

The Cavalier-Paladin was an original creation of Gary Gygax himself, first appearing in Dragon Magazine #72, before appearing in the 1985 Unearthed Arcana splatbook and then a third time in Dragon #148. It was basically a super-Paladin, which is exactly why it earned so much hate; although it came with a bunch of powerful class traits, it combined the Paladin's already obnoxious mechanically-enforced Lawful Stupidity with an even more party-annoying obnoxious bit of "enforced fluff" - the actual need for the player to act like a Leeroy Jenkins, but constantly charging off to fight enemies, even when smarter approaches were required. Even the author of the revised version from Deragon #148 admitted that the Unearthed Arcana Cavalier was "overly complicated to play".

Needless to say, the Cavalier-Paladin made no friends in D&D audiences and, aside from 4e Essentials including a Paragon Path named the Cavalier, it has been consigned to the dustbin of history ever since.

The other version of the Cavalier has been around since Advanced Dungeons & Dragons. Essentially, it's a Fighter variant, revolving around increased skills at fighting from the back of the steed. From a kit in second edition (The Complete Fighter's Handbook, Player's Option: Skills and Powers) to Prestige classes in 3 (Sword & Fist) and 3.5 (Complete Warrior) to a Martial Archetype (class branch) for the 5e Fighter, it's fairly stable. Unlike the Cavalier-Paladin, though, it's not mechanically forced into being obnoxious... but it's still never really taken off.

Why? Because the Cavalier has always been a one-trick pony; good at fighting from steed-back. Put them in a situation where they can't ride their steed - like, say, crawling in your stereotypical dungeon - and they're basically a worthless half-fighter.

Pathfinder eventually took a shot at making a Cavalier class of its own, the first time it's been a full class since AD&D. Although probably more popular as serving as the foundation for the Pathfinder's Samurai class, it does manage to be more effective than Cavaliers have traditionally been - this is because it manages to be a two-trick pony, with a focus on teamwork through the use of challenges and features that bolster allies.