I admire
@BRICKBURN and
@MarkB, and if you are so wired that is the best advice you have had. I am not. I have to see it and "feel" the proportions. That means I am constantly going up and down ladders at our place whenever a new beast or painting comes home to stay (and patching holes!)
You did good on the design - you have tall wall space. As
@Scott CWO suggests, vary heights. I saw enough battalion formations to last a lifetime in the military. A friend has a regiment on display in his great room. Large ones dress right dress on one wall and two rows (!) on another. It really looks terrible, but what can you do.
For instance I would have fully used the space above the fireplace (?) by raising the Rhino above the two buffalo - or vice versa. It would provide depth and proportion. I personally think the rhino would be better high, because it is larger which lengthens its perspective a bit compared to the flanking buffalo.
Mix some art or memorabilia in with the mounts. A leopard or buffalo painting or prints, Masai shield, large masks, crossed lion spears, etc always add interest. There are a couple of threads on trophy rooms here. Browse them. Try to ignore the architecture, but instead focus on how the ones that please you exploit space, proportion, and color.