The Rattail was designed by the Baron Council and used to judiciously to take control of the swamp world Alamar, where the early settlers had an adundance of radioactive chemicals at their disposal and thus an easy way to mass produce simple irradiation grenades. These grenades proved surprisingly to be more useful than the standard gauss rifles in keeping the local fauna, and to an extent, flora, at bay and their settlements safe.
However as the Baron council grew in wealth and power they realized the weapons that had protected their early settlements from harm could prove extremely effective in fighting against the zerg, and thus more effective grenades were developed. Now few living creatures can last long against a sustained barrage from a determined squad of Rattails.