Chitha (nicknamed psyolisks by the terrans) is a zerg sub-strain of the hydralisk modified with xel'naga essence. They were created by Abathur years after the End War. Abathur did not consider them true zerg, as they were equal parts zerg and xel'naga. Abathur utilized them to aid in a scheme to destroy a similar breed, the adostra, sparking the One-Day War.
Before she ascended to become a xel'naga, Sarah Kerrigan informed Overqueen Zagara of a peaceful species that could host the genetic essence of a xel'naga. Zagara tracked down this species and had Abathur modify some of them with xel'naga essence recovered from Ulnar, creating the adostra. The adostra were intended by Zagara to use their psionic abilities to seed life across the galaxy. Abathur quietly opposed this idea, wishing for the Swarm's wars to continue to further spur their evolution. To this end he secretly created the chitha as well as the adostra from the remaining xel'naga essence, and gave them powerful psionic abilities to combat the protoss and disrupt communications and psionics. To aid in commanding them he also modified the queen Mukav.
When Emperor Valerian Mengsk and Hierarch Artanis came to Gystt to see what the adostra had done to the planet, Abathur positioned numerous chitha in the hives where the adostra were kept in sleep. The chitha attacked the terrans and protoss the two commanders sent to observe the hives, while also baiting them into destroying the adostra. Abathur's wish was not only to re-ignite tensions between the races, but to destroy the adostra that he saw as abominations.
When Valerian deduced Abathur's gambit as the situation unfolded on the surface, he realized Abathur and Mukav were commanding the chitha from a leviathan, and set a trap for him by arranging for Zagara and a force of terran and zerg warriors to board the same leviathan. Abathur was exposed and he attacked the group using the chitha. The chitha were apparently all killed in the ensuing battle, but all three races agreed to keep an eye out should any others reveal themselves.
The chitha are based on the hydralisk's genetic template, but are much smaller, slightly smaller than zerglings. Rather than the dark brown coloration of most zerg, chitha were light brown with red highlights on their legs and claws, with triple lines of bright red spots starting between their eyes and running all the way to their skulls and the center of their backs. Their appearance and their red dotted carapace are designed to imitate the appearance of the adostra.
Unique among the zerg breeds, they are psionic due to their xel'naga essence. Their psionic abilities let them stun protoss and psionic terrans with mental attacks, and also allow them to control lesser zerg breeds to an extent. Enough chitha concentrating their powers together can disrupt terran communication signals, interfere with protoss warp prism signatures, prevent use of the protoss warp matrix, and overwhelm a protoss warrior to the point of near total incapacitation. However, they are unable to block communication via protoss psionic boosters. Though the chitha are based on the hydralisk, they forsook the hydralisk's spinal attacks, instead relying on stunning enemies with their psionic abilities to allow them to get close and use their bladed arms to attack. Chitha are susceptible to high heat and flame based attacks.
So as to keep their existence secret from Zagara, Abathur did not create them with a connection to the zerg hive mind, instead relying on vocalized commands to order them.