Not too long ago, I watched a match on stream from #ggChronicle and saw Alistar Headbutting the blue buff golem over a wall. Based on that observation, I want to know if Alistar can Headbutt other objectives over walls, such as Dragon?
Alistar cannot headbutt Dragon nor Baron over the wall, for one simple reason. Baron and Dragon are now both immune to Debuffs and Crowd Control effects, so no displacement spell will ever move them.
Alistar might be able to headbutt the double Golems over the thin wall behind them, but I don't think the walls near Wraiths and Wolves are thin enough to do the same with them.
Nowadays, no body can do anything versus Baron or dragon that would move they position, Debuffs, knock up and Crowd Control effects (Besides kite the dragon a little away from his spot). A couple of years ago that was possible.
Check those links:
As you can see, in both of those videos was possible, but they are very old, and RIOT changed it a while ago.
As for Alistar. He can throw even champions over walls, not only Golem and others monsters. But the push is short, basically you can push over walls where you can flash too. Not big walls/forest.