As of Generation V, ten Pokemon may know it at encounter. I'll provide their type and name, in case you want a specific Pokemon in particular.
Magmar, a Fire Type
Slugma, a Fire Type
Magcargo, a Fire and Rock Type
Magby, a Fire Type
Magmortar, a Fire Type
Hitomoshi, a Ghost and Fire Type
Ranpuraa, a Ghost and Fire Type
Shandera, a Ghost and Fire Type
Meraruba, a Bug and Fire Type
Volcarona, a Bug and Fire Type
Also, these Dream World Pokemon will know it.
Ponyta, a Fire Type
Rapidash, a Fire Type
Moltres, a Fire and Flying Type
Heatran, a Fire and Steel Type
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the Pokemon that have flame body are mostly magmar,torcoal or slugma
use action replay or a pokemon with the "flame body" ability
Use a Pokemon with the ability magma armor or flame body (Slugma's ability usually)
You have to walk around with it. It will hatch faster if you have a Pokemon with the ability Flame Body.
In battle, the ability Magma Armor prevents freezing. Outside of battle, if the Pokemon with Magma Armor and an egg are in the party, Magma Armor will halve the number of steps required for an egg to hatch.The following Pokemon have Magma Armor: Slugma (dual ability with Flame Body), Magcargo (dual ability with Flame Body) and Camerupt (dual ability with Solid Rock).