Dragons - In the Third Age, the Dwarves were constantly being harassed by powerful creatures that drove them from their homes.  In Moria, it was the great Balrog; and in the North is was dragons.

"But Thorin I his son removed and went into the far North to the Grey Mountains, where most of Durin's Folk were now gathering; for those mountains were rich and little explored.  But there were dragons in the wastes beyond; and after many years they became strong again and multiplied, and they made war on the Dwarves, and plundered their works.  At last Dáin I, together with Frór his second son, was slain at the doors of his hall by a great cold-drake.   Not long after most of Durin's Folk abandoned the Grey Mountains.  Grór, Dáin's son, went away with many followers to the Iron Mountains; but Thrór, Dáin's heir, with Borin his father's brother and the remainder of the people returned to Erebor." ~The Return of the King (Appendix A)