Alone, Malacoda appear as long worms covered in tumorous growths. They have a circular maw ringed with long, thin teeth. The maw is accented by two large mandibles that hang off the head. They are eyeless, with only smooth plates covering the top of their head. Their tail ends in another orifice ringed with similar looking teeth. They are very long, and powerful enough to crush a helicopter without noticeable recoil damage.
In Revelations, a number of these creatures inhabit what is surmised to be an infected whale. The creature's appearance is grotesquely modified from any known living whale and even shares some physical attributes with shellfish. The Malacoda host is massive, easily swallowing up most of the foredeck on the Queen Zenobia. It is a dark green colour, with a large tear down its spinal column. From this tear, several deformed vertebrae can be seen. The head tapers from the body and is filled with irregular, wickedly sharp teeth. This creature's mouth is remniscent of a carnivorous saurian, with the addition of a strange white growth inside the mouth of the creature that functions as a weak point. This creature also seems to lack eyes.
The creature's body ends in what appears to be myriad, small claws that make a singular foot. These claws are used for grasping the deck of the Zenobia during the battle. It has what appears to be a flat, rounded tail that covers the front of the ship.
One of these parasites infected a large organism, possibly a whale or giant squid and grew to a gigantic size. It attacked BSAA agents Chris Redfield and Jessica Sherawat when they were attempting to board the Queen Zenobia but they managed to repel it by destroying its tentacles.
When exploring the Zenobia's laboratory, Jill Valentine narrowly avoided dying when the Malacoda broke the glass, allowing the T-Abyss virus to flood the chamber. Fortunately, she had injected herself with a vaccine moments before she became exposed to the virus and survived.
When Chris and Jill attempted to evacuate the Zenobia after the self-destruct countdown had been triggered, the Malacoda destroyed the rescue helicopter and revealed its full form, a gigantic whale-like organism with multiple respawning tentacles. With the help of rocket launchers dropped by Kirk Mathison, they managed to subdue the beast.
After boarding the second helicopter, the Malacoda revived itself and the two agents were forced to defend themselves from its tentacles. They managed to weaken the Malacoda enough to fire a missile down its throat, killing it for good.