Trolls are a Scandinavian speciality, and among our oldest monsters. In most Nordic countries they were evildoers who eat humans, especially children, or tried to trick us.
But not in Denmark. Here they were noting more than magical neighbours who borrowed stuff from the farmers, and if the farmers treated them well, they were rewarded with gold or luck. They also loved children, so the easiest way to make friends with a troll was to be nice to their kids, and it was said that they took care of the human babies that parents abandoned in the woods.
The reason why the Danish trolls were friendlier than the others is most likely because we already had a creature that ate children and used magic to trick us: The valravn.
Valravns were ravens that had eaten from the dead human bodies on the battlefield and thereby gained human intelligence and magical powers. But they could only fly doing the night, and to break this curse they had to eat the heart of a child. If they did that, they would gain the power to turn into a knight or a half raven/half wolf beast that could move around doing the day.
Therefore they were always on the lookout for children and would make deals with women to get their firstborn. They were overall considered quite evil.
So here is a Danish troll who has found a baby in the woods and was on his way to take it home to his family, when he ran into a valravn who is now trying to talk him into giving it the child.
A song about a valravn [link]