If we consider a more general view, in some regions where reindeer are native, baby reindeer might be part of a herd managed by indigenous people. The cast (assuming you mean group) of baby reindeer would grow up learning from the older reindeer in the herd. They'd learn how to forage for lichens, which is a major part of their diet, and how to migrate to different areas depending on the season. Their real story could involve survival in harsh Arctic or sub - Arctic climates, dealing with snowstorms and limited food resources during the long winters.
In fanfiction, the author might be using this as a creative device to add humor and character to their interactions. Deadpool calling Spider - Man 'baby boy' could also be a way to show Deadpool's dominance in their relationship. He's a more chaotic and unpredictable character, and by using this term, he's asserting his own brand of crazy over Spider - Man.
I'm not entirely sure. It might be a real story if it's based on someone's actual encounter with a baby reindeer in the wild or on a reindeer farm. However, it could also be a fictional story made up for entertainment, like in children's books or movies where baby reindeer are often depicted in magical or anthropomorphic ways.