I'm sorry, 已然形 doesn't exist in modern Japanese.已然形 is, I believe, the one of the only six Japanese verbal stem forms that indicates potential/ability.
In classic Japanese, you use “能(よ)く～す” or “～す可(べ)し” ... to express “can do”, “すべからず”,“する能わず”“せられず” “～得(え)ず”to express “can not”.
可能形 is the de-facto name for 書ける 言える 見れる 話せる …(sometimes includes できる 食べられる... depending on the context)
But I don't think it's a good name, do you?
可能形 is usually not used to express 可能性, 可能性 is usually not expressed using 可能形.