From the 2020.02 edition of the core rules.

Dragoon

Dragoons are models with a mounted stat card and a dismounted stat card. They begin the game mounted but can become dismounted during play.

When a mounted dragoon suffers damage, apply that damage to its mounted dragoon damage boxes. When all these damage boxes are marked, the dragoon becomes dismounted instead of disabled. Damage points in excess of the mounted dragoon’s remaining unmarked damage boxes are not applied to its dismounted damage boxes. If a dragoon is dismounted while advancing, it cannot continue to advance; if a dragoon becomes dismounted during its activation, its activation ends. Remove the mounted dragoon and replace it with the dismounted dragoon model (see “Replacing Models,” p. 33). Apply effects that were on the mounted dragoon to the dismounted dragoon. When the dismounted form replaces the mounted form, the dismounted form is not considered to have entered an area and does not trigger effects triggered by entering that area. Once this replacement is complete, any further damage the dragoon suffers will be applied to its dismounted dragoon damage boxes. The dragoon is disabled when all its dismounted damage boxes have been marked.

Rules Clarification : Dragoon      (Edit)

• Dragoons become dismounted instead of becoming disabled, so you can't trigger anything that works on disabled (such as Tough) when you kill their horse.
• If a dragoon is dismounted while overlapping another model (such as an Incorporeal one), then you have to rewind to the dragoon's last legal position before resolving the dismount. (Infernal Ruling)
• When a Dragoon is dismounted, even though you replace the physical model, it is considered the same in-game model for the purposes of whether the original attack hit/damaged it (for stuff like Chain Attack or Drag). For instance, you could hit the mounted model with your first attack, damage it enough to dismount it, then resolve Drag vs the dismounted model. (Locked thread)