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):