As you well know, Finntroll incorporates elements of black metal, joik, and humppa.
Humppa is described by Wikipedia as:
. . . a type of music from Finland. It is related to jazz and very fast foxtrot, played two beats to a bar. Typical speed is about 250 to 280 beats per minute. Humppa is also the name of a few social dances danced to humppa music.
You can hear the humppa elements in this performance of Nattfödd:
Eläkeläiset, based on my Googling, is another popular Finnish humppa band that plays mostly pop/rock covers, like Europe's Final Countdown.