Can't advance to do Eric's quest for the dragon tooth
 

Need to bring Eric 5 ancient bears, 15 wolves and 10 vikings to get the dragon tooth. Cannot get pass the magic wall or do anything else to do this quest.

You don't need to go past the wall in order to complete this quest; Vikings are easily available from the main castle while bears and wolves are always available from a trainer on top of the (snowy) mountain, right about where the bridge is. Just hire the creatures and bring them to Eric.

Collect those units then.

Wolves can be bought at the soothsayers hut, vikings in the castle and bears from the cabin on top of the snowy mountain. You can leave your own units in the castle to make room for them.

Edit: aww too slow.

What units are vikings, as I do not seem to find them and the quest fails for me as well. I tried Axe throwers and zerks, but he does not like them.

The Viking is one of your original units, and they are in the castle for purchase.

Viking is your main melee unit, looks like a lower level of Berserker