I think the song describes him as a “pissed ant”. The song as a whole is about philosophers being drunks, and the word “pissant” has nothing to do with drunkenness, but simply means a worthless, contemptible person. However, in Britain (as many Americans probably do not know, even if American Dopers do), “pissed” is a common slang word for drunk. Furthermore, I believe “pissant” is primarily an American word, with relatively little currency in Britain. (Collins Dictionary flags it as a US word, and my Firefox spellchecker does not recognize it as a word at all.)
The fact that many internet lyric sites say “pissant” probably just reflect the fact that they are based on transcriptions by Americans, who can make no sense of “pissed ant” and so substitute something similar sounding that makes a sort of sense to them, despite being at odds with the theme of the song.
I admit I can’t very satisfactorily explain why he is referred to as an ant (apart from the fact that they needed a rhyme). I am prepared to entertain the hypothesis that the Pythons themselves (or Eric Idle in particular) were somewhat confused, perhaps having heard the word “pissant” somewhere, and having misunderstood it to be an idiomatic expression, “pissed ant”, used to describe a drunken person.