A theory on Reddit suggests that the rat from Avengers: Endgame, the same one that released Ant-Man from the Quantum Realm at the beginning of the movie, is the same rat that appeared in the first installment of 2015's "Ant-Man".
Ant-Man, played by Paul Rudd, got trapped in the Quantum Realm after Thanos snapped his fingers in Infinity War. Only in Avengers: Endgame, Ant-Man is released by a rat that accidentally presses the buttons on a console.
Now, according to the theory shared by the user "MCU", the rat from Avengers: Endgame is the same one that appears in the first installment of Ant-Man, precisely when Scott Lang uses the suit for the first time. He falls into the drain and ends up being threatened by a "giant" rat.
It would be incredible if this were true, but the Marvel community denies that this is the case for time reasons. There are nine years of difference between the movies of Ant-Man and Avengers: Endgame. A rat lives up to a maximum of three years. So it would be impossible for it to be the same.
Theory: The rat that released Scott out of the quantum realm in Endgame is the same rat in Ant-Man. from r/marvelstudios