I based my 'Lupogryph is male' thinking on the innate aggression of the character. A female character would tend to be more wary when dealing with something utterly new like this, and take more time to come out of the shadows, probably even warning Leaf more on what to expect. She would attempt communication on more levels, probably wagging her tail in welcome or seeking to sniff his hand to get his smell more firmly fixed in her mind. She would pay attention to what might be alarming him, and react accordingly.
She would be both more elusive and less dramatic. And she would pay more attention to the way she herself is feeling. That last might even be the most important.
'Guys think, girls feel' might be an oversimplification, but there is some truth in it regardless.
Your Lupogryph is very 'thought-focused', if you will - ergo, male.
Not that I write a lot of female characters or anything <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />