I'm actually not familair with a... Mareep, but the others I know of. Horsea and Magikarp are water types, and Dratini is a DRagon type so... Above all try to keep the egg warm and when Magikarp hatches try to have a LOT of trust and understanding with him/her because you really don't want the evolved form mad at you, trust me.
Anyway if you're near a fire when camping or in a Pokemon Center try to be clsoe enough that the egg is warm but not too close that you boil the egg.
Re: [video]
I'm actually not familair with a... Mareep, but the others I know of. Horsea and Magikarp are water types, and Dratini is a DRagon type so... Above all try to keep the egg warm and when Magikarp hatches try to have a LOT of trust and understanding with him/her because you really don't want the evolved form mad at you, trust me.
Anyway if you're near a fire when camping or in a Pokemon Center try to be clsoe enough that the egg is warm but not too close that you boil the egg.