Visceroids are the results of uncontrolled, irregular Tiberium mutation.
Visceroids were disturbing creatures that sometimes form when organic life dies to Tiberium exposure. As flesh breaks down after death, it has the potential to mutate into a visceroid, a crude Tiberium-based life-form. Visceroids often appeared to be blob-like masses of flesh that sometimes had a few recognizable features from those they once were left over within their structure, and were highly erratic in nature. Some types were even capable of combining their biomass to essentially make a larger visceroid. One of the first mutated creatures to appear, the visceroid was encountered as early as the First Tiberium War.
The Scrin Corrupter, or rather, its Tiberium spray, was noted to create Visceroids on occasion, an unintended side effect of the BC weapon. This does not occur in Kane's Wrath, despite the Corrupter's tooltip stating otherwise.
A Visceroid was also seen attacking an Annihilator in a Tiberium field in Cologne.
In Kane's Wrath, Visceroids are moving insta-rank crates. Any human non-epic infantry gain near instantaneous Heroic status. Venoms and other craft move up to the Heroic rank when they at least kill three Visceroids.
After a Corrupter mutates enemy infantry into Visceroids, the Visceroids would go berserk and attack the Corrupter that first mutated it, but does no damage. Eventually, they would 'calm down' and accept commands from the Scrin Foreman that mutated them.