Control slots were implemented with publish 16 in order to curb rampant spawn hogging by unethical tamers with multiple dragons or white wyrms. They are an effective way of limiting the number of high-level pets a tamer can control at any given time, while still allowing for the diversity of hunting with a pack of lower level creatures.

You are only able to control a number of pets based on your pet control slots. You must have the required control slots available for your mount (ethereal mounts require one control slot). Every player has 5 pet control slots, the expanded version of your health bar will show you how many are currently occupied:

Most pets only require one slot, with the following exceptions (current status as of 07/04/02):

• 2 control slots: unicorn, ki-rin, nightmare, drake, imp, hellhound leader.
• 3 control slots: white wyrm, dragon, golem (changed from originally 4 slots)
• Hirelings (human NPCs) were changed from originally 4 slots to 2 slots.
• The predator hellcat leader and frenzied ostard were originally planned to take 2 slots
  but were permanently adjusted to require only one.

For completeness' sake: a summoned daemon takes up 5 control slots, a fire elemental 4, a water elemental 3, an earth or air elemental 2.