The Neuro Toxin is a powerful venom used to fight or protect against creatures. It kills them slowly by decreasing their Hit Points. Some Units have the ability to use these in many ways. The only human known to use it is Rex Chance .
With the creation of the Sigma Technology, the new creatures needed to be handled by some way. But they were more powerful than average animals for using conventional weapons. That is why, at some moment within their development, they developed the Neuro Toxin as a way to contain them in case of protocol breaching.
The Neuro Toxin has been used by Rex Chance to deal damage to creatures.
Rex discovers Neuro Toxin darts on his father's lab, which he can fire using the same dart gun he uses to collect Genetic Samples from wild animals. Upon exiting the lab, he is attacked by a Tigging Mantis ( wich he previously found on his first visit), which can be dispatched fairly easily by using the newfound ability.
The Neuro Toxin darts are also vital in The Wicked Sky Witch, to prevent Velika's scouts from alerting her to your location. The first few scouts come long before you can get any AA towers or ranged attack creatures built, so you can use Rex to shoot them down.