As others have already pointed out, it's an archaic/poetic form. It's hardly ever heard or used anymore, except maybe in such old proverbs/sayings/song lyrics, and even then with only certain adjectives (like "kallis" or "paras" -> patjaksensa
parahin / matto levitellään) - so you do not really need to master it for everyday usage
Rakkain (from
rakkain) would then likewise be ->
rakkahin...
In some traditional songs you might come across "kallehin" -> kallein (the dearest,
not the most expensive -as in 'kallis ystävä'- although some might argue that these two may intertwine

)
Then again, in some dialects that "h" is still prominent.
Notta me veretähän sua turpahan
(not genuine ostrobothnian but something like that)