MASSIVE SPOILERS FOR AVENGERS: ENDGAME! IT WAS AWESOME!
Henry was a rat; they lived about a year or two depending on what kind they are. They do not exist to do much. They were born to eat; possibly mate and hide from predators who saw them as snacks or humans who would want to exterminate them or use as lab experiments.
There was nothing to say that Henry's life was any different.
Henry was about middle aged at a year old; he had heard legends of the Decimation; where half of all animals turned to dust. His father had told the story which he heard from his father who experienced it firsthand. The rats were weirded out by this event but on the bright side it meant fewer predators.
Plus the humans were too busy panicking.
One day he came across a van in a storage facility; he was walking about the controls trying to find anything to eat. He ended up meeting his destiny there.
In a one in a billion trillion chance he managed to set off the controls of an advanced Quantum Realm machine in the van which set free Scot Lang from the Quantum Realm.
Once being set free and learning of the Decimation; he reunited with his daughter Cassie Lang. However it was later did he realise this could lead to Time Travel as for him he had only been trapped for 5 hours.
That domino effect led to the restoration of the half of the universe that had been wiped away. Rats and predators where weirded out to find themselves returned to an unfamiliar Earth.
As the Thanos of 2014 sat down in utter defeat and sorrow watching his entire army turn to dust; unable to do anything other than wait for his turn; he had no idea it was because of Henry the rat that his efforts were defeated.
As the world celebrates the return of the Snapped and try to fix the world; they start mourning those lost in these events and celebrating those who had made this possible.
However no one knew the role that Henry the Rat had played in all this; a crucial one at that. Only Doctor Stephen Strange ever knew as he had seen the role Henry played when viewing the 1 possible future where they won through the Time Stone. However the millisecond after viewing his role he immediately forgot about him; because he was a rat so why should he even contemplate even giving this rodent another thought. He didn't know or even care that Henry had a name.
Life went on and Henry passed on; but among the rats that lived... there was now a legend that there was one of their own who was crucial in the reversal of the Decimation. One who was in the right place at the right time to make the ultimate difference.
Not a lot of them believed it; some did and some dedicated their lives to finding the evidence of his existence and his role in all of it.
The problem was that they were rats so not in much position to find evidence and since their lifespan was so short they did not have time to even make even a triple way decent search.
So ends the story and legend of Henry the Rat; the ultimate unsung hero of the Marvel Cinematic Universe!