The Baby Enderman is a new type of Enderman (in a mod), that spawns in the Enchanted Island biome. They are visually smaller, do not attack players, and cannot teleport, unlike regular Endermen. However, they are dangerous as they will steal the player's items and call their relatives when attacked.
Baby Endermen will occasionally target a random nearby player, track them down, steal a random item (except armor and currently held item) and run away.
If the player attacks them, they will select nearby Endermen as their relatives, and have them teleport and attack the player. Even though they will only do it once and killing their relatives will make the babies unable to protect themselves, it is still recommended to use indirect damage such as water or fire.
It is also possible to give the Baby Enderman a different item and in case he likes it more, he will drop the original item. Each Baby Enderman has his own preferences and will like some types of items (metals, food, plants, ender items, etc.) more than other types.