As described in Why is Pikachu wearing a Santa hat and what does it mean?, there is a limited time promotion where wild Pikachu can be wearing a Santa hat. Additionally, if they are evolved into Raichu, they too will be wearing a Santa hat.
Pikachu's pre-evolution is Pichu, and the most recent update announced a Johto Holiday and the ability for Pichu to hatch from eggs
Professor Willow has discovered Togepi and Pichu hatching from Eggs! Starting later today, Trainers will have the opportunity to hatch these and several other Pokémon that were originally discovered in the Johto Region in Pokémon Gold and Pokémon Silver video games. These are the first of more Pokémon coming to Pokémon GO over the next few months.
Can a Pichu wearing a Santa hat hatch from an egg?