The Larva morphs into most Zerg units. Larvae are automatically produced by a Hatchery, Lair, or Hive as long as there are not at least 3 Larva at that building already, and can be created by the Queen's Spawn Larva ability.
The Zerg Larva is the most basic of Zerg creatures and has sustained it's role throughout several millennia. Zerg Larvae, which look like giant maggots, contain, embedded in their DNA, the genetic code for all creatures of the Zerg. If the Overmind issues a command, the Larva will begin the metamorphosis cycle. When a new creature is needed, an individual Larva can enter a pupal state (an egg form) and begin building upon the DNA of the specific Zerg creature at a rate unsurpassed by any other living thing. After a short maturation period, the new Zerg creature bursts out of it's egg and is now ready to be commanded.
A premature hive mind will have only the most basic genetic codes for Zerg creatures, such as the Drone or Zergling, but as it evolves and develops new structures, it's library of genetic strains is expanded and newer creatures are able to spawn.