I had this revelation today in the train home from the Hospital (no, I am not hurt or the like, I just needed to be there today, in case you are wondering).
Threehorn is a racist to a certain extend, no argument about that. But that is only a piece of his problem with the other grown-ups. The MAIN part of said problem that causes the constant trouble is that
he has trouble adjusting to no longer being in charge!!!
I always assumed that he had been the Leader of the Threehorn-herd, what with how he acted, but only today I realized what that meant: for almost his entire adulthood HE was the one that told them where to go, HE was the one that looked for new food,
HE was the leader/protector/what-have-you.
Now, in the Great Valley, he is suddenly thrust in a position where he has to not only continuusly share that control over his herdmembers (indeed, there is no true Leadership-position anymore at all), but the main sort-of-Leadership-position has gone to someone else (Littlefoot's grandfather) to booth, who is not even a Threehorn. (<= racism hello

)
And so not only does he suddenly have to discuss everything with other dinosaurs he never really liked because of their species, but he also needs to get them to his side to get things done his way. And he sucks at that: what logically follows are arguments that go nowhere because he is unable to accept compromise. But another major problem is that he
has to accept someone else telling him what to do at times. His changes over the series are not so much him becoming nicer, but more him finally accepting that he is no longer the main-leader for those around him and learning to live with that.