The level a creature unlocks at is determined by its rarity.
Common creatures unlock at level 1
Rare creatures unlock at level 6
Epic creatures unlock at level 11
Legendary creatures unlock at level 16
Unique creatures unlock at level 21
Common creatures unlock at level 1
Rare creatures unlock at level 6
Epic creatures unlock at level 11
Legendary creatures unlock at level 16
Unique creatures unlock at level 21
Apex creatures unlock at level 26