Regarding the essence level system, the level increase occurs independently for each subspecies. Foxen Sionnach/Vulpuss levels up separately, for example.
Next, each species requires a certain level of one (or several?) of the 3 stats (libido/fertility/aptitude). The stat varies depending on the species.
And from a certain essence level, some traits of a certain quality are added to the prerequisites (like swift 2 for the foxen). This part is bugged because some traits are impossible to obtain so early.
In short, this part of the game is messed up, and unless you want to investigate for each species, use the cheat code: "Essence #" (1 to 5) to bypass this part.
Enjoy your game!