The giant Pistachion is, as one might have guessed, an abnormally large Pistachion, spawned from the next generation of Pistachions from their second invasion in "Fungus Among Us" and "The Phineas and Ferb Effect". He is the last, ginormous obstacle Milo, Dakota, Cavendish, Orton and Diogee face before being able to leave Lard World (which has become a human detention center during the invasion) in search for Professor Time near the end of "Fungus Among Us".
The giant Pistachion is notably disguised as Lardee Boy, Lard World's mascot, and is for some reason guarding the entrance to the sewers nearby the lard stream from Lard World's lard disposal.
The giant Pistachion is just as animalistic in nature as in appearance, tending to roar like a beast over verbal communication. Like an actual animal, it gains one's attention when they make loud sounds, attacks his enemies when taunted, and uses brute force in order to free himself. Despite his beastly nature, he is in fact able to speak like all Pistachions can.
He is ordered to attack any humans like Milo, and does so when his cover is blown. He can be easily angered when pistachios get destroying in front of his face, as he is still a sentient pistachio at heart.
The giant Pistachion is, like most Pistachions, a mutated pistachio tree with a slightly opened pistachio nut for a head, brown wooden limbs, and leaves covering his torso. Unlike other Pistachions, however, this one is more animalistic and not as much anthropomorphized, with a hunched stance more akin to a gorilla than a human being. This Pistachion is also much larger than any other Pistachion, easily being over 3–4 meters tall and its limbs as thick as tree trunks.
Unlike most of its brethren, who have their foliage cover only their chests and pelvises, the giant Pistachion's foliage grows over the entire body. One other distinct feature are its pupils; regular Pistachions have a red and orange glow in their eye sockets that act as pupils, but the giant Pistachion has black, oblong pupils within this glow. Uniquely, this is the only Pistachion with this feature.
The giant Pistachion marks his debut in "Fungus Among Us", in which he is encountered at the end of the lard drain of Lard World's lard disposal. Milo, Dakota, Cavendish, Orton and Diogee reach the entrance to the sewers via a boat, but it is blocked by a grate in a grease trap. After a few attempts to break out, the group is stumped, when the Pistachion, disguised as Lardee Boy, approaches. Due to his disguise, Milo mistakes him for the actual, reliable, friendly lard mascot, but as he asks him for help, the Pistachion tumbles over, dropping the Lardee Boy face in the process and revealing his true nature. When Milo sees the Pistachion face inside the Lardee Boy costume, he flees as the giant Pistachion bursts out of his disguise, revealing his true size. After trying to attack the rest in the boat, he decides to chase Milo over a narrow bridge.
Eventually, Milo gains an idea how to get out. He orders Orton to tie one end of a chain to the grate, while he tricks the giant Pistachion into locking one of its paws in an empty grease spout, which has the other end of the chain inside. Milo does this by gaining the Pistachion's attention with an airhorn and angering him by destroying some pistachios he has with him. The trick works, as the Pistachion steps his front paw in the grease spout, getting it and the chain stuck around it. As Milo and Orton reunite in the boat, the Pistachion tries to grab them, but is restrained by the chain. He uses his strength to break himself free, ripping the grate from the grease trap in the process, creating an opening for Milo and co. to escape. Milo, Dakota, Cavendish, Orton and Diogee speed through the opening as the giant Pistachion chases them again, but as the grate locks back into the grease trap, the giant Pistachion is forced to hang outside the grease trap as the group escape, when he mutters what he can do about it.
The giant Pistachion reappears in "The Phineas and Ferb Effect". During the giant battle between the Dakota paradoxes and the Pistachions at Lard World in the fourth act, Melissa, Zack, Orton, Cavendish and Dakota try to run away as they encounter Derek, wanting to recapture them to use the M.U.L.C.H. machine against them. He unleashes the giant Pistachion, who rises up from a trap door and has raised his fist up in the air, ready to crush them. However, as he looks around and observes the onslaught between the Dakotas and the Pistachions, the giant Pistachion has an eye-opener, wondering what has become of him and realizing he has been held underground for so long he forgot compassion completely. He puts on a baseball cap, packs up a knapsack on a stick, and leaves Lard World and the battlefield, out to better himself and help people. Derek isn't all too pleased by this and scolds that they don't need him anymore.
After Professor Time and Orton have slain Derek in 1955, everything regarding the Pistachions has been undone; from the Lard World takeover, to humans turned into Pistachions, to the Pistachions themselves. This includes the giant Pistachion, whose last moments were serving soup to a customer at a soup kitchen and wishing him a wonderful day.
“Eh, what are you gonna do?”