The most obvious answer is “I Am the Walrus”, but “Come Together” and “Glass Onion” also contain the word “walrus” in their lyrics. In “Come Together”, it’s mentioned in the line “he got walrus gumboot”.
In “Glass Onion”, “walrus” is mentioned twice in this verse:
I told you about the walrus and me, man
You know that we’re as close as can be, man
Well here’s another clue for you all
The walrus was Paul
All three of these songs were chiefly written by John Lennon. Incidentally, on his 1970 solo album ‘Plastic Ono Band’, the song “God” contains the line “I was the walrus, but now I’m John.”