I think the problem you're having is this:

A hidden resource is added to a region, just like any other resource (e.g. gold). Cities are not added to resources. Here is a useful tutorial to help you out for reference:
https://forums.totalwar.org/vb/showt...dden-resources)

Quick summary:

1. Add your hidden resource to those cities where you want your special unit to be recruitable in descr_regions.txt:
Code:
Britannia_Inferior 
Eburacum 
britons 
Brigantes 
232 41 198 
stone, coastal_forest, lead, dogs, pigs, guineapigs
5
5
2. Define the hidden resource in export_descr_buildings.txt:
Code:
hidden_resources rome, sparta, guineapigs
3. Now add the hidden resource as a requirement to your special unit:
Code:
recruit "guineapigrider" 0 requires factions { roman, } and hidden_resource guineapigs
Remember to delete map.rmw

Good luck