If adding a new centaur designed to be a significant threat increase over the existing ones, they should generate with good (if non-magical) armor and a good weapon, which is frequently a lance. If they have a lance, they will use it to joust you.

If you are a centaur (via polymorph or some variants’ centaur race), you can joust things without needing to ride another monster.

Since polearms are designed for use against mounted enemies, they get bonuses versus all u and C monsters. Also mounted enemies, if in a variant that has monster riding.

Centaurs can use lances as if they were riding.