I think some character development has definitely occurred with Mr. Threehorn (though most of it seems to have happened before Charles Grosvenorówho, frankly, seems to have little respect for continuity, or, in recent years, viewers’ intelligence :rolleyesótook over). In his first appearance, he obviously didn't want Littlefoot to come near Cera, and, implicitly, considered longnecks stupid. In the second movie, he is still against the idea of Cera playing with non-threehorns, but when he is briefly seen witnessing her doing so (immediately before the song "Peaceful Valley"), he does not intervene. In LBT III, he is quick to attempt to prohibit Cera from playing with Littlefoot after accusing the latter of wasting water (though it may be worth noting that he was already riled up because of the water crisis, and likely felt humiliated when he was accidentally splashed), but by the end of the movie, he seems to accept the value of cooperation, and that yelling is far from an ideal parenting method. He also appears to have developed a greater respect for longnecks, as, following the fire, he openly admits that Grandma Longneck is right, and thanks her. After this movie, we never see him attempt to prevent Cera from playing with the others again.
In V, Mr. Threehorn continues to show contempt for longnecks (though, realistically, this is a trait he would probably never lose), but he goes along with Grandpa Longneck’s decision to keep the herds together (if he argued at all, it was not shown). Eventually, however, he decides that the herds should split up. This could be taken as the first example of continuity ignorance on Grosvenor’s part, as Mr. Threehorn again seems to be favoring segregation (evidently having forgotten his lesson from III), but it could just as easily be argued that he was influenced by the combined stresses of lack of food and water, days of traveling under harsh conditions, and conflicting opinions on which direction to take (the same conditions, incidentally, that caused the gang to split up in the first movie).
(This is all I have for now. My brain has apparently gone dry.

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