It gets me angry how absurdly rare Munchlax is in D/P/Pt as a wild Pokémon

It only appears on honey trees, which take six hour after spending honey on it, but also, Munchlax can appear on only one tree in the game, which is decided from a calculation made from the player's secret ID, so without anything to access the game save, you can't know which one for sure

But the worst is: you only have a 1% chance of having a Munchlax on the right tree. ONE PERCENT. That's absurd.