The amount of units that any faction can build during a time period has been limited. Many units for factions have even been limited to specific provinces within their homelands. Generally this happens when a unit is superior to its generic equivalent. I went through the MM_Full_unit_prod using the Gnome editor and made spreadsheets for each faction so I would know what is going on. Checking the buildings only tells you what it can build currently, not what it will be able to build in the future, unfortunately. It is confusing and more units go obsolete in 3.12 than the regular game. Column AC of the v3 Units Text has the Factions that can build each unit and AD has the regions they can be built in.

For Mercia the only units available would be,
In Early: Peasants - Muster Field, Woodsmen - Town Watch, Spearmen - Spearmaker, Fyrdmen - Spearmakers Workshop and Longbowmen - Bowyers Workshop+Swordsmith.
In High: Fyrdmen, Billmen - Town Militia, Longbowmen, Welsh Longbowmen - Bowyers Guild+Swordsmiths Worksop and Muntators(Spanish Jinetes) - Horse Farmer+Spearmakers Workshop.
In Late: Fyrdmen, Billmen, Muntators, Welsh Longbowmen and English Longbowmen(60 Sherwood Foresters) - Master Bowyer+Swordsmiths Guild+Armourers Workshop.

As you can see no Cavalry other than Muntators. All English heavy cavalry is only available in their French possessions.