Where Kant and later writers talk about concepts, Plato and Aristotle and medieval writers talked about forms in somewhat analogous ways. Neither concepts nor forms have the immediate unproblematic accessibility that is claimed for Cartesian mental representations or Lockean ideas or medieval species. Where concepts or forms are to the fore, we are generally in discursive territory.
Leen Spruit has documented that the middle ages also saw a huge variety of doctrines of so-called “species”, both perceptible and intelligible, which in one aspect were mental representations, some resembling phantasmata in Stoicism, some seeming rather like Cartesian mental representations or Lockean ideas. These were generally considered to be contents immediately accessible to the mind. I tend to think a lot of these were probably associated with what Brandom calls two-stage models of representation, where representings are considered to have an immediate intelligibility that representeds lack.