In the old age of lore, Temmie's were once proud, charitable Lion Dragons Boss Monsters, but with the passing times, the Tems descended from Boss Monsters to be better integrated into Monster society, turning into the lovable balls of hyper we see them as today, though some of their lineage is still seen today, with their ample amounts of money and excellent craftsmanship.
((Do not take this as fact, i just came up with this.~))