* A habitat: A large forest, a sprawling savannah, or even a bustling city can hold a lot of wild animals together, even though it's not something that can be locked.
* A food source: A plentiful food source like a watering hole or a fruit-bearing tree can attract and hold many animals in the area.
* A shared instinct: A shared instinct like migration can bring many animals together for a period of time, even though they are not "locked" in place.
Let me know if you'd like to try another guess!