If you have a female Mudkip, leave it at the day care center with a male from the following list:
Aggron, Aron, Azumarill, Clamperl, Corphish, Corsola, Crawdaunt, Exploud, Feebas, Golduck, Gorebyss, Grovyle, Horsea, Huntail, Lairon, Lombre, Lotad, Loudred, Ludicolo, Marill, Masquerain...