This picture shows what was possible back in the old Tibia.
People could summon Dragons and stuff and simply logout. After that the monsters would go wild and attack everyone.
It was possible to summon up to 8 monsters. As you see in the Picture. But since you got no Exp from Summon-Kills back then the only use of them was for PKing.
You ask yourself why you can only summon 2 monsters today and why they disappear after you log?
Here is the answer.