Easter Eggs and Hidden Details in REANIMAL.
Five Characters

More like a detail than a Easter Egg, but here is the clear view of the five characters looking down on you in the beginning of the game.
Covering Eyes

If you light up brother's face with the light in sister's hand, he will cover his eyes.
Rest/AFK

Don't touch the mouse and the keyboard for a short time and watch the characters sit down and take a break.
- Turn the light off first.
Suitcases from Little Nightmares

The suitcases you find throughout the REANIMAL game are the same as the suitcases in the Little Nightmares games.
- The first two Little Nightmares games are made by the same developers of the REANIMAL.
Executed Shadow

After using the Handcar to get to the other side of the railway, stop and go right through a open train to find a box. Opening that box shows shadow of the character you saw before getting executed with a shot to his head while his hands are tied.
Fox

Right before entering the cave with the Handcar on the railway, look up your top right corner screen to see a Fox watching you.
Five Rabbit Statues

Check out the hidden room in your left at the end of the tunnel you enter with a handcar for 5 rabbits representing the five kids you see in the beginning of the game. [https://wikigames.net/reanimal-easter-eggs/#Five_Characters]
Stay together to stay alive (Co-op)
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In Co-op, if you keep the characters too far apart, the Sister will have difficulty breathing, and the Brother will be hanged.
- The interesting part is that the Sister was found in the water, and the Brother has a noose around his neck.