While searching the map, some wild Pokemon have sprites other than HGSS (or BW, in the case of Unova-only Pokemon) from older Pokemon games. These are referred to as Retro Pokemon, or Retros. Retros have a smaller chance of appearing than HGSS sprites. Retros are broken up into several categories, all based on the games they appeared in; Green, Red/Blue, Yellow, Gold, Silver, Crystal, Ruby/Sapphire, Emerald, FireRed/LeafGreen, Diamond/Pearl, Diamond/Pearl Frame 2, Platinum, Platinum Frame 2, HGSS Frame 2, BW.

HOW APPEARANCE RATES WORK

The mechanism used to determine what pokemon you run into in any given area works like a random number generator (RNG).
The RNG first calculates for randomly-appearing pokemon, such as the V-Day Vulpix or starters with the Seekers (avail. in the header shop).
If the number comes up negative, the random event doesn't appear, and the RNG moves on.
Next, it determines what pokemon is going to show up, depending on the area you're in. When you're on the map, at the bottom of the page is a link called "View Mapdex;" clicking this will pull up the appearance rate of pokemon for the area you're in.
Then, it checks for retros. You have a 1 in 100 chance to get a retro - any retro. A negative outcome causes it to check for a frame2 HGSS sprite, which is a 1 in 10 chance.
A positive outcome, however, yields the following rates in Kanto, Johto, Hoenn, and Sinnoh:

50% chance of it being Platinum (and then 33% chance of it becoming Pt2)

30% chance of it being D/P (and then 33% chance of it becoming DP2) 5% chance of it being FR/LG 5% chance of it being R/S (and then 33% chance of it becoming Emerald) 3% chance of it being Silver (and then 33% chance of it becoming Crystal) 3% chance of it being Gold (and then 33% chance of it becoming Crystal) 2.5% chance of it being Yellow 1.4% chance of it being R/B 0.1% chance of it being Green

**In Unova, the Retro rates for NON-Gen V Pokemon are as follows: **

20% chance of it being B/W

40% chance of it being Platinum (and then 33% chance of it becoming Pt2) 20% chance of it being D/P (and then 33% chance of it becoming DP2) 5% chance of it being FR/LG 5% chance of it being R/S (and then 33% chance of it becoming Emerald) 3% chance of it being Silver (and then 33% chance of it becoming Crystal) 3% chance of it being Gold (and then 33% chance of it becoming Crystal) 2.5% chance of it being Yellow 1.4% chance of it being R/B 0.1% chance of it being Green

Note that if the particulat Pokemon being checked is only available in later generations, the chance for the earlier generation sprite defaults to HGSS (or B/W, for Gen V ONLY)

WHAT DO THESE NUMBERS MEAN?

If you aren't a mathematically inclined person, these numbers can be confusing. 50% sounds like a good rate, but how much of 1% is that?
That's where the absolute appearance rate comes in: these are the rates of a particular retro coming up overall, not just within the 1 in 100 retro chance.
Note: these rates do not account for the GB Player, which doubles the retro appearance rate. To find these, just double the decimal.

Italicized data are applicable to Unova ONLY

HGSS has an 89.1% chance, or roughly 9 times out of 10 HGSS2 has a 9.9% chance, or roughly 1 time out of 10

B/W has a .2% chance –1 in 500

Platinum has a .33% chance –1 in 300 Unova: .277% chance – roughly 1 in 370 Platinum2 has a .167% chance –1 in 600 Unova: .133% chance – roughly 1 in 752

Diamond/Pearl has a .2% chance –1 in 500 Unova: .134% chance – roughly 1 in 746 Diamond/Pearl2 has a .1% chance –1 in 1,000 Unova: .066% chance – roughly 1 in 1515

FireRed/LeafGreen has a .05% chance –1 in 2,000

Ruby/Sapphire has a .033% chance –1 in 3,000 Emerald has a .017% chance –1 in 6,000

Silver has a .02% chance –1 in 5,000 Gold has a .02% chance –1 in 5,000 Crystal has a .02% chance –1 in 5,000

Yellow has a .025% chance –1 in 4,000 Red/Blue has a .014% chance – roughly 1 in 7,143 Green has a .001% chance –1 in 100,000

Credit for this page goes to VDex user Cykotyks and Shirogane for helping them. This data on this page was originally posted Here