Forests, Mountains :
Level 1 - Young Wolf, Boar, Ghost
Level 2 - Wolf, Dire Boar, Elder Ghost, Goblin
Level 3 - Dire Wolf, Goblin Warrior, Young Hillbeast, Bear
Level 4 - Elite Goblin, Orc Scout, Hillbeast, Nightmare
Level 5 - Goblin Shaman, Orc Warrior, Giant, Rock Golem, Blood Thooth
Level 6 - Goblin Voodoo Mage, Orc Elite, Horror of Night, Were Wolf
Some of those mentioned creatures are not spawning everywhere. For example Giants and Rock Golems are spawning only in caves and mountains. But like orcs and goblins can create camps. And also orcs and goblins can be quest creatures. Like Goblin Voodoo Mage is a very special and rare creature that it could be very likely a quest creature. The steps that I made levels 1-6 could be increased when your hero gets a level. This way wouldn´t all the time travel to a new unexplored place. Also Were Wolf exists only at nights and it comes only when Horror of Night is near a Blood Thooth. Also made those colors to help you. Following a color you can see which unit becomes which unit.
Lakes, Rivers, Swamps :
Level 1 - Crab, Murloc
Level 2 - Angry Tortoise, Murloc Hunter
Level 3 - Giant Crab, Dire Tortoise, Murloc Tide Guard
Level 4 - Murloc Assassin, Naga River Crusher
Level 5 - Murloc Shaman, Naga Ocean Cryer
Level 6 - Naga Elite Warrior, Thrill Skreamer
Farms, Ruined Villages :
Level 1 - Skeleton, Young Field Raider, Rogue, Ogre
Level 2 - Skeleton Archer, Field Raider, Thief
Level 3 - Skeleton Soldier, Bandit, Field Muckraker
Level 4 - Skeleton Mage, Dire Field Raider
Level 5 - Skeleton Elite Soldier, Ogre Mauler
Level 6 - Banshee, Ogre Magi, Giant Field Muckraker
Like that. And I could explain abit that Banshee. Normally differen kind of creatures would fight with other creatures. Like Skeletons kills Ogres and so on. But banshees takes control of some unit and the target unit becomes neutral to all kind of units and it gains also more attack damage and other stats. And also banshee herself won´t dissapear so a single banshee can control like 10 units at the same time. They won´t attack until banshee or themselves are attacked.