Out of the Abyss features a group of Duergar that are mounted combatants called Kavalrachni.
They're pretty easy to adapt to any race that you want because they only have this unique ability that sets them apart from any old Duergar:
Cavalry Training. When the duergar hits a target with a melee attack while mounted on a female steeder, the steeder can make a melee attack against the same target as a reaction.
This is perfect for the "Lancer" feel, as it synergizes surprisingly well while the creature is mounted, here's why:
Normally, a controlled mount has only a limited number of actions, according to PHB 198:
... [a mount] moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge.
With Cavalry Training, a rider can order its mount to make one attack as a reaction if the rider hits with his attack first, essentially breaking the above rule.
The above can be further amplified if the mount has Trampling Charge, like a Warhorse (MM 340):
Trampling Charge. If the horse moves at least 20 feet straight toward a creature and then hits it with a hooves attack on the same turn, the target must succeed on a DC 14 Strength saving throw or be knocked prone. If the target is prone, the horse can make another attack with its hooves against it as a bonus action.
Because a mount and rider share their turns, the reactionary attack from Cavalry Training can activate Trampling Charge, essentially allowing a 3-hit combo (Lance, Hooves, Hooves).
CR Change. Out of the Abyss says that a Kavalrachni has the same stats as a Duergar but with this trait, its CR increases by 1. By my estimates, and looking at similar creature abilities, I would add the potential damage from the mount's highest-damage melee attack to the rider's DPR.
Is this overpowered? Not really. So many things have to go right for the rider and mount:
- The mount has to move 20 feet straight toward the target
- The rider needs to hit with his lance
- The mount needs to hit with its hooves
- The target needs to fail the strength save
- The mount needs to hit with its hooves again
At any point in that process one of those aren't fulfilled, this glorious combo cannot be attained. If it does, though, it's pretty devastating... Kind of like getting hit by couched lance, and a charging warhorse.