Why don't you mention Кащей-бессмертный (Kaschei the immortal) and Змей Горыныч (Gorynych the serpent)? They are evil creatures but, nevertheless, very famous (or infamous) folklore characters.

Кащей-бессмертный
http://en.wikipedia.org/wiki/Koschei


Змей Горыныч
http://en.wikipedia.org/wiki/Slavic_dragon


Also there is Кот-Баюн:
Unfortunately, there's only German version available in the Wikipedia:
http://de.wikipedia.org/wiki/Bajun


Wait, isn't Cat Bayun is also a Scandinavian folklore character?
Here's a better illustration (just as l imagined):