In the 1997 film Starship Troopers, the giant brain-like alien that is captured is the Brain Bug.
The Brain Bug is an intelligent member of the Arachnid species, the insect-like aliens fighting humanity in the film. Unlike the Warrior, Tanker, Plasma, and Hopper bugs, it is not primarily a frontline combat creature. Its large, exposed brain-like body represents the Arachnids’ strategic intelligence and ability to direct their war effort.
After an ambush on Planet P, Colonel Jenkins tells Johnny Rico that the military deliberately sacrificed Rico’s unit to confirm the Brain Bug’s existence. Rico is then ordered to return to the planet and capture it. During the mission, the Brain Bug kills Zander Barcalow by consuming his brain, apparently absorbing his knowledge.
The creature escapes the immediate assault but is eventually captured by Sergeant Zim, who is shown triumphantly holding it. A propaganda broadcast announces that scientists will study the captive alien to learn the Arachnids’ secrets. “Brain Bug” is the film’s canonical name; the other bug names describe different Arachnid forms, not alternate names for this creature.