Blitzen is a red enemy that only appears during the Jolly St. Nick event.
A Rain D. variant which is more resistant to knockbacks (less knockbacks), can knockback the player's cat units, is (strangely) a red unit, and has a small boost in attack and health. However, it attacks slower and moves slower.
The only difference between fighting this enemy and Rain D. is its ability to knockback your cats and the fact that it is a red unit. Using any Fish Cat variant (Crazed, Island, etc.) will be able to kill this unit quite quickly.