Alpha-sapphire: The flame that burns at the tip of its tail is an indication of its emotions. The flame wavers when Charmander is enjoying itself. If the Pokémon becomes enraged, the flame burns fiercely.Y: From the time it is born, a flame burns at the tip of its tail. Its life would end if the flame were to go out.X: The flame on its tail indicates Charmander’s life force. If it is healthy, the flame burns brightly.
Prof. Lotus' Notes: Charmander being the most popular of the 3 starters is attributed to more than just it's evolutions, because in 1999, most players didn't really know, nor care about the evolutions when starting. Charmander is actually another example of good video game design, where a psychological technique known as the Center Stage Effect, the selection of products based on the one in the middle - deeply routed into our psyche. The reason for this is Game Freak wanted players to not just rely on one super powerful Pokemon, but to actually go out there and catch more Pokemon that would cover their needs, instilling a behaviour early on.