ATmega XTAL input capacitance and crystal load capacitance
Crystal pins are not I/O pins, and 10pF is a guaranteed maximum not a nominal capacitance.

Usually 5pF will suffice for a crystal pin plus stray capacitance with reasonable layout (as short traces as reasonable, possibly over a ground plane). However, the AT90USB1286 datasheet does not recommend load capacitors of less than 12pF so 10pF total would seem to be marginal (you'd get 11pF if the input+stray was 5pF). 



If you don't care about the exact frequency, 12pF or even a bit higher would probably be okay with a "10pF crystal", it would just shift the nominal oscillation frequency very slightly. Otherwise you can pick a different crystal.