It is a fairly tempting conclusion to draw that Timby did not at first believe Nixy about the moose infestation. This would be a correct conclusion. It is also a tempting conclusion to draw that this is because of the fantastic nature of Nixy's claim. This would be an incorrect conclusion.
It is reasonable to infer from remote incidents in Timby & Nixy's history together that Timby's incredulity was based more in experience, e.g. the time Nixy got Timby to ride horseback through a den of buttered chickens. Timby has learned that when Nixy posits a challenging scenario, it might be a dishonest bid to distract Timby from his well-guarded role as the most insane character present.
Or perhaps it is possible that we're wrong about all that, and Timby was simply eager to withhold mercy from a bunch of moose that he presumed much more mellow than they actually were.
"Timby! There's a moose infestation in your sock drawer!
"No wait! You don't wanna do that! They're raising a family!"
RING RING "Joe's pest control, how–"
RUMBLE "AAAAAA! HELP, THEY'RE STAMPEDING!" CLICK!