mon-slot.md"
Last-Modified: Sat, 21 Mar 2026 21:47:32 GMT
Expires: Tue, 18 Mar 2036 21:47:32 GMT
ETag: "02d6663d1c50c672540bc9dc6e1b3fc79e635ded"
This tutorial is for how to add more variety for wild Pokémon. As an example, we'll add an eighth slot for wild Pokémon in tall grass, and a fourth for wild Pokémon in water.
## Contents
1. [Increase the relevant constants](#1-increase-the-relevant-constants)
2. [Add more probability slots](#2-add-more-probability-slots)
3. [Add more wild data](#3-add-more-wild-data)
## 1. Increase the relevant constants
Edit [constants/pokemon_data_constants.asm](../blob/master/constants/pokemon_data_constants.asm):
```diff
-NUM_GRASSMON EQU 7 ; data/wild/*_grass.asm table size
-NUM_WATERMON EQU 3 ; data/wild/*_water.asm table size
+NUM_GRASSMON EQU 8 ; data/wild/*_grass.asm table size
+NUM_WATERMON EQU 4 ; data/wild/*_water.asm table size
```
If you've seen the [data/wild/\*.asm](../tree/master/data/wild/) files before, it should be obvious that `NUM_GRASSMON` is the number of slots for Pokémon in tall grass at a particular time of day, and `NUM_WATERMON` is for Pokémon in water at any time.
## 2. Add more probability slots
Edit [data/wild/probabilities.asm](../blob/master/data/wild/probabilities.asm):
```diff
GrassMonProbTable:
mon_prob 30, 0 ; 30% chance
mon_prob 60, 1 ; 30% chance
mon_prob 80, 2 ; 20% chance
mon_prob 90, 3 ; 10% chance
- mon_prob 95, 4 ; 5% chance
- mon_prob 99, 5 ; 4% chance
- mon_prob 100, 6 ; 1% chance
+ mon_prob 94, 4 ; 4% chance
+ mon_prob 97, 5 ; 3% chance
+ mon_prob 99, 6 ; 2% chance
+ mon_prob 100, 7 ; 1% chance
WaterMonProbTable:
mon_prob 60, 0 ; 60% chance
- mon_prob 90, 1 ; 30% chance
- mon_prob 100, 2 ; 10% chance
+ mon_prob 80, 1 ; 20% chance
+ mon_prob 95, 2 ; 15% chance
+ mon_prob 100, 3 ; 5% chance
```
These are just the cumulative probabilities for each possible slot.
## 3. Add more wild data
This will be tedious. If you edit `NUM_GRASSMON`, you need to update the wild data for *every* map in [data/wild/johto_grass.asm](../blob/master/data/wild/johto_grass.asm) and [data/wild/kanto_grass.asm](../blob/master/data/wild/kanto_grass.asm); the same goes for `NUM_WATERMON` with [data/wild/johto_water.asm](../blob/master/data/wild/johto_water.asm) and [data/wild/kanto_water.asm](../blob/master/data/wild/kanto_water.asm). Make sure that all the quantities are correct.
For example, we increased `NUM_GRASSMON` from 7 to 8, so here's how Route 29's wild data could change in [data/wild/johto_grass.asm](../blob/master/data/wild/johto_grass.asm):
```diff
map_id ROUTE_29
db 10 percent, 10 percent, 10 percent ; encounter rates: morn/day/nite
; morn
db 2, PIDGEY
db 2, SENTRET
db 3, PIDGEY
db 3, SENTRET
db 2, RATTATA
db 3, HOPPIP
db 3, HOPPIP
+ db 3, MARILL
; day
db 2, PIDGEY
db 2, SENTRET
db 3, PIDGEY
db 3, SENTRET
db 2, RATTATA
db 3, HOPPIP
db 3, HOPPIP
+ db 3, MARILL
; nite
db 2, HOOTHOOT
db 2, RATTATA
db 3, HOOTHOOT
db 3, RATTATA
db 2, RATTATA
db 3, HOOTHOOT
db 3, HOOTHOOT
+ db 3, POLIWAG
```
And we increased `NUM_WATERMON` from 3 to 4, so here's how Cherrygrove City's wild data could change in [data/wild/johto_water.asm](../blob/master/data/wild/johto_water.asm):
```diff
map_id CHERRYGROVE_CITY
db 6 percent ; encounter rate
db 20, TENTACOOL
db 15, TENTACOOL
+ db 20, CORSOLA
db 20, TENTACRUEL
```
Don't miss any tables, or you could encounter glitch Pokémon, prevent encounters completely, or cause other weird bugs.
Anyway, that's it! As long as `NUM_GRASSMON` and `NUM_WATERMON` match the lengths of `GrassMonProbTable`, `WaterMonProbTable`, and the many individual maps' wild data tables, then you can have as many slots as you want.
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