Why 8...Na6 was played in the immortal draw? From the other question: Is there a &quot;perfect&quot; game? , an interesting game came up:

[fen ""]
[title "Hamppe-Meitner, Vienna"]

1.e4 e5 2.Nc3 Bc5 3.Na4 Bxf2+ 4.Kxf2 Qh4+ 5.Ke3 Qf4+ 6.Kd3 d5 7.Kc3 Qxe4 
8.Kb3 Na6 9.a3 Qxa4+ 10.Kxa4 Nc5+ 11.Kb4 a5+ 12.Kxc5 Ne7 13.Bb5+ Kd8 14.
Bc6 b6+ 15.Kb5 Nxc6 16.Kxc6 Bb7+ 17.Kb5 Ba6+ 18.Kc6 Bb7+ 1/2-1/2


My question is, why is 8...Na6 considered the best move? References to GM analysis would be great. 

Stockfish says 8...Nc6 is better--in fact, 8...Na6 is not even on the top 10 recommended moves. And I can see even in the short term that the knight on c6 instead of a6 might be nice: consider ...Na5+.
I talked to National Master Dan Heisman about this game and according to him, the answer is really very simple:

At the time, black thought that 8...Na6 was the best move.  Black's intention was to control b4 and get to c5.  After that both sides played almost perfectly which resulted in a draw.

It is also possible that the game was actually made up, but this is hard to prove 140 years later.

Black probably would have played something else besides 8...Na6, but with no computer analysis at the time, it was thought to be best.