When discussing finding Pokémon in the Wild, Pokémon World Online does not use a labelling system to determine which Pokémon are easiest or hardest to find. Instead, we use a "Tier" system, where Pokémon are rated based on the average amount of time a player is expected to extend to encounter a Pokémon.
Key to spawn rates:
Tier 1, marked as #1 - Easiest to find, will appear in under 20 minutes on average.
Tier 2, marked as #2 - Slightly more difficult. Should appear on average, 20 minutes to 1 hour apart.
Tier 3, marked as #3 - 1 to 5 hours average time between spawns.
Tier 4, marked as #4 - 5 to 15 hours average time between spawns.
Tier 5, marked as #5 - Anything over 15 hours average time between spawns.
The time estimates given for each Tier represent the average amount of time a person would have to invest to find a Pokémon of that tier in a given area. This does not mean that a Pokémon listed as "in one hour" will be found in one hour, nor does it mean you always have to wait one hour to find another. What it means is, if you spend one hour in this map (and it doesn't have to be all at once), you will on average see at least one of this Pokémon.
A better way to think of it is: If a Pokémon is listed as appearing "once per hour", and another "once per day", you are about 24x as likely to find the first than the second at any given time. Of course, luck plays a factor too, so maybe you might find the rarer Pokémon first sometimes, or maybe it might take a little longer to find one. Treat these guides as estimates only.
Butterfree is set to take an average of 7 minutes to find in Johto's Hidden Crag. This means on AVERAGE butterfree should show up every 7 minutes, however it is still based on luck - so you might find 3 in a row, or go 30 minutes between sightings. The same goes for whether the average time is 7 minutes, 7 hours or 27 hours. Luck is always a factor, we just provide averages to give you an idea of what to expect.
Imagine that you roll a bunch of dice every time you encounter a Pokemon in the game. Each side of these several different dice determine different things.
For example, the dice that determines if a Pokemon you see is shiny has 8,192 sides if you don't have membership. So, when you roll that dice every encounter, the dice has to land on ONE particular side for that Pokemon to appear shiny.
So let's say you get an encounter and roll those dice. You see a normal Rattata, which means your dice didn't land on that one specific side that lets the Rattata be shiny. You run away and encounter something else. This time you see a shiny Pidgey! That means it landed on that one side of the dice that says "make this Pokemon shiny." Hooray! Next time you roll it again...maybe on next encounter you will see something shiny, maybe not. The point is that what you see in Pokemon World Online is determined by chance that you as a player of the game cannot affect.
Now, let's apply that basic concept of dice to Rarity Tiers and its descriptions. Imagine if billions of people over time roll the dice that determine which Pokemon will appear, for hours on end. How many rolls will it take most of them to see a particular Pokemon? Somebody might find that map's Tier 5 after rolling dice for 10 minutes and another person might need over 100 hours, but most people out of those billions will see it within the given time-frame above. That's how the averages for tiers work; it accounts for everyone who encounters anything on that map, not just a single person's experience. If a billion people roll a dice, what they normally see as a group becomes the average. The average doesn't guarantee when you will find something. The game isn't "broken" if it takes you a bit longer to see a Tier 5 than that average, it just means you're an outlier.