Chomper and Ruby haven't yet Rinkus and thus only known him from the tales by their friends. Especially colorful description of the flyers was by Cera, who said that they were nasty, stupid and dangerous.
However as they met with the Longtail personally, Ruby and Chomper have seen him from a completelt different angle. Rink turned out to be quite a friendly flyer who liked to raise children. At that time, he already was taking care of his daughter and two adopted sons.
Of course, since the first meeting, Rinkus himself was quite careful and distrustful of the gang as he was protecting the chicks under his care. However his concerns were pointless as the little flyers quickly found common ground with the gang, which allowed Rinkus to reassess his previous opinion of the Leafeater children.
The Flyer himself found commonality with Ruby and Chomper, whom, just like Draigar, looked at him like an old uncle, a role which Rinkus has accepted a long time ago) And the Flyer hasn't yet met a Sharptooth who can speak the tongue of the Leafeaters. So it can be safely assumed that Rinkus is no more surprised than the gang that entered the lands of Backwings.