If I were to pre-spawn monsters, let's say cows and then the game fills with players, do they give 1 player exp and easy to kill or do they give 8 player exp and easy kill?

8p games 8p exp

Monsters spawn 1-2 screens away from characters. So if you made cow portal with 1 person in game and didn’t enter. No cows have technically spawned yet. You could then bring in more people and then enter and 1-2 screens away (in all directions) cows would have spawned. They would be at the experience rate at which there are players in game.

If you entered cow portal prior to people joining and ran through the whole cow level and people entered into game, they would all be spawned as it was 1 player in game and thus low xp given.

1player exp