A "littermate" refers to one of the puppies or kittens born in the same litter. When animals reproduce, they typically give birth to multiple offspring at once, and these siblings are known as littermates. They share the same mother and are usually born within a short time frame, making them genetically related. Littermates often grow up together, which can influence their social behaviors and development. While having a sibling can provide companionship and play opportunities, it can also lead to challenges in training and socialization if they are raised together without proper guidance.