Hi, hepburn,
Gen Sir Frederick Horn is an interesting suggestion.
Of note, he and Gen Lysons both had the GCB, Legion d’Honneur, Crimea Medal, Sardinian Crimea Medal and the Turkish Crimea Medal as well as the Order of the Medjidie, 3rd Class. The only difference here, as has been mentioned before, is that Gen Lysons had three Crimea clasps whereas the sitter in the painting (and Gen Horn) has four.
So, could he be the sitter ? Maybe. The one niggle I have here is the first medal in the row - what appears to be the Queen Victoria 1897 Diamond Jubilee Medal with Clasp ‘1897’. If that is correct, then that would exclude Gen Horn as (according to Wikipedia) he died in 1894.
I agree that the two men do not look entirely dissimilar so, if the 1897 Jubilee medal is wrong, you might be correct. That said, you mention the Wikipedia picture might be a ‘younger’ version of him. As he became a GCB in 1889, he was approx. 84 or older when that painting was done of him - so I would think he should look almost exactly similar to the sitter but the resemblance is not what I would call a strong match.
As mentioned before, if we ever get a close-up of the original medals from the original poster that might help resolve this matter.
ATB
Simon