Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...d5/5. exd5/5...Na5/6. Bb5/6...c6/7. dxc6/7...bxc6/8. Be2/8...h6/9. Nf3/9...e4

= Two Knights Defence =

9... e4
With the pawn under a attack, challenging the White Knight again is an obviously useful move here, as it gains a tempo.

Theory table
.
 * 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Na5 6.Bb5+ c6 7. dxc6 bxc6 8. Be2 h6 9. Nf3 e4

Statistics
Estimated next move likelihood Ne5 99%, Ng1 1%