Helminth Charger is an Infested, Kubrow-like companion incubated with the Helminth Cyst. Its Proboscis precept pulls enemies in, Trample damages enemies in its path while strengthening itself, and the Strain mod set can spawn cysts that create Maggots.
You need Howl of the Kubrow, a Kubrow Egg from Grineer Forest Kubrow Dens on Earth and a built Incubator Power Core. Unlike other Kubrows it also requires a Helminth Cyst: use Nidus or Nidus Prime in the Orbiter's Helminth room to inject, wait about a week for the cyst to mature, and then select Begin Incubation. Keep the infected Warframe equipped and be patient, as the cyst takes about a week to mature.
Helminth Charger was added with Update 19.5 (December 2016).