PMU 5, or more commonly known as "Stone Age PMU", is one of the many "Pokemon Mystery Universe" versions. PMU 5 was created in 2008, running on an engine called "Eclipse". PMU 5 was founded by Linkachu.
PMU 5 had extremely similar basic gameplay in comparison to its descendants; the player would move with the arrow keys and attack with the Ctrl button. In addition, the stat system was wildly different - rather than being based on the stat system in the main games, every level gave the player three stat points to spend on various stats, unless the player had a special stat point-effecting item. Moves could be accessed through the menu on the top right. Players were able to recruit Pokemon, just like in later versions of PMU. and players could buy "TMs" or find them via dungeon. Howeverm, at the time, these were called "Spells" and only certain "TMs" could be taught to certain Pokemon. Evolution did not exist in this version - there was no method to do so until later versions.
Every map was 15 tiles wide by 20 tiles tall. The tiles were a mix of Pokemon Mystery Dungeon tiles and tiles from other games, such as RPGmaker and the default tiles from Eclipse, just like the later versions.