Laughing Mare is a white female horse firs seen in "Darkness Falls".
She used to be a normal horse. After the magic from the Crystal Cavern was unlocked and began to spread across Dreamland, some of it got in the water from the well. Laughing Mare drank from this water and was given the same ability to laugh as Laughing Horse.
The two horses immediately fell in love now that they could both laugh. At the end of "Goodbye Bean", they have started a family and move into the cave of Malfus, much to the latters annoyance.