So the problem with the Zerg when it comes getting something close to there size is that there aren't any know insects that have achieved the size of something that big.
I know studies have shown that insects have achieved the size of 2-3 meters or (6.5 - 9.8 feet) in times of Anicent Earth, scary but still not close to a Overlord from StarCraft.
An Overlord is Overlords retain the thick outer shell of the Gargantis, and it changed little in the assimilation process. Their exoskeletons are strong enough to resist a lightning strike.
Overlords: with the correct growth stimuli, [they] can carry other zerg within hollows in their hides. In this form they become deep-space transports; the importance of their function is underlined by the sheer number of overlords found accompanying zerg forces. As spacefaring creatures, an overlord's carapace pressurizes and seals whenever the creature flies through vacuum. Two species of unidentified symbiotic organisms seem to regulate these functions, though Dominion scientists have been unable to obtain any living samples—these organisms die within seconds if removed from their host overlord. Due to their need to support many different strains of zerg at once, overlords can sometimes exhibit spontaneous adaptive mutations in order to improve their own efficiency.
Definition taken from http://starcraft.wikia.com/wiki/Overlord.
However exoskeleton animals developed in the ocean and then started to come onto land. So if were trying to get a creature that is close to be anatomically correct of I would suggest the Lion Jelly Fish in terms of size when it would be close.
The largest recorded specimen found washed up on the shore of Massachusetts Bay in 1870, had a bell with a diameter of 2.3 metres (7 ft 6 in) and tentacles 37.0 m (121.4 ft) long. Lion's mane jellyfish have been observed below 42°N latitude for some time in the larger bays of the east coast of the United States.
So my idea would be to have this jellyfish mutate in the water and develop an exoskeleton from mutation. This works as well since I believe Overlords use helium-filled gas sacs and a weak telekinetic psi-ability for lift and motive power. This helium is generated through an efficient respiratory system distributed throughout the overlord's carapace. The excess helium is stored in thick sacs that contract and expand through rudimentary pulses, allowing overlords to regulate altitude and propulsion at will. They move quite slowly however (if you are looking for it to float that is)
Lion's Mane Jellyfish
Size Compassion for all Zerg