Prince Jake had a dog named Homer. When Ax had been masquerading as Prince Jake, he'd learned how to treat dogs. When the "gud dog" was finished smelling his hand, he reached out and gently scratched behind one of its ears. Andalite hands were weaker than human hands, but the dog would likely still enjoy it.

< I am an Andalite, > he said. < My home planet is very, very far away from Earth. >

Assuming that was where the dog was from. But most of the people on board seemed at least familiar with Earth if they weren't from there themselves.

< My name is Ax, > he added.