Johan is much less surprised by the rat king's animosity towards him than he's letting Ginko believed. Animals--magical or otherwise--do not like him very much. This was not always the case; it predated his time as a vampire, but especially since then...

His feigning ignorance (and innocence) is about much more than just Ginko's gullibility, though.

"No... you may be right. If I approach slowly enough from behind, it might move away from me." He shrugs a little, almost carelessly. "It couldn't hurt to try it."