Standard DOSBox setup the game comes with in native version is not working. I installed flatpak DOSBox and Flatseal through Discover.

Use Flatseal to adjust DOSBox flatpak permissions - in "Other files" add new path "/home/deck".

Then save this launching file anywhere (e.g. in your home folder):

#!/bin/bash
cd "/home/deck/.steam/steam/steamapps/common/Prehistorik 2"

export LD_LIBRARY_PATH="/home/deck/.steam/steam/steamapps/common/Prehistorik 2/dosbox/libs/x86_64"

flatpak run com.dosbox.DOSBox -conf "/home/deck/.steam/steam/steamapps/common/Prehistorik 2/dosboxPrehistorik2.conf" -conf "/home/deck/.steam/steam/steamapps/common/Prehistorik 2/dosboxPrehistorik2_single.conf" -noconsole

Save file with ".sh" extension and set it as executable (right click on file -> Permissions -> check "Is executable"). Then copy file location (right click on file -> copy location) and change launch options as shown above using correct path to this file.

The game supports gamepad but you might actually want to play with control mapping a bit to fully enjoy playing the game as e.g. menu screen doesn't work with gamepad and also jumping is not comfortable with default setup.