In Back to the Future Part III, it's briefly shown that Buford Tannen had something against Seamus McFly, having previously told him not to come in the saloon. Now this is basically to repeat the scene from each film where a Marty hears a Tannen shouting "Hey, McFly!" only for it to turn out he's referring to somebody else (well Marty did know he was referring to his son in the second film), but isn't it a little weird that there's not really any scenes with Seamus and Buford? We never see or hear anything about Buford bullying Seamus, and Buford doesn't seem to care (if he notices at all) that Seamus is at the saloon during the climax. Of course in the film he's more focused on Marty and Doc. With the new Back to the Future comic books having stories about things not seen in the original trilogy, it would be great to see a Seamus McFly and Buford Tannen story showing their feud.
Though Seamus McFly is a bit different from George McFly and Marty McFly Jr. He doesn't seem to be a wimp, just somebody who doesn't really believe in violence or care what others think of him, so it was probably easy for him to get pushed around. It also seems like the majority of townspeople in 1885 didn't care if people called them cowards (though they were when Marty finally stood up to Buford, and when Marty decides he doesn't care anymore, the people in the saloon warn him about what people will say about him). That could be a generation thing, but also, the majority of characters in 1885 were adults, while in 1955, 1985, and 2015, most of the people Marty deals with (and those who Biff and Griff deal with) are closer to Marty's age, where they are more likely to care about if somebody is a chicken.