Both of them had a previous wife and children, whom they lost most of (Mr. Threehorn kept Cera) and both remarried and later had a child. Also, both of them are really grouchy but improve over time (Mr. Threehorn took considerably longer than Mannie.)
Also, Mr. Threehorn seems, somewhat, to have befriended Chomper, even though he may well have lost his wife and kids to Sharpteeth. And Mannie forgave humans, despite losing his family to them, and even defended a human from sabertooths.