Special troops are troops that belong to a major kingdom but they are not within the kingdom's troop tree. They are considered elite troops of the kingdom, but only some lords or monarchs can train them.
Each kingdom has at least one, and these are:
Empire special troops:
Ravenstern special troop:
Fierdsvain special troop:
D'Shar special troop:
Sarleon special troop:
Jatu special troops:
Mystmountain special troop:
Note: only D'Shar Djaha Archer and Marleons Heavy Cavalry can be recruited through rescuing them from prisoner packs or by making them prisoners and then recruiting them in the camp option. Marleons Heavy Cavalry will be added occasionally to Marleons' garrison, no matter who owns the city.
As of 3.9, the player can gain access to most of the troops on this list via asking certain companions for troops (see here) or having certain companions go out to collect right to rule. In addition, successful gambles with Ramun the Slave Trader can get you small quantities of some of these units. The only troops on this list the absolutely cannot be obtained are the Jatu and Mystmountain ones.