It almost seems as though Ruby doesn’t fear Sharpteeth as much as the herbivores or feel quite as threatened. I mean, she seems to know about Sharpteeth and them losing their teeth and that they will grow back, etc. Either she spent some time with Chomper’s parents and learned that, or she’s actually talked to Sharpteeth before. As she promised Chomper’s parents that she’d look after him, and, as I’ve seen no indication that Chomper’s parents know a word of Leaf Eater, she must know the Sharptooth language. Also, in the Spooky Nighttime Adventure Episode, she seems to know that the story is told by Sharptooth parents to Sharptooth children.
Furthermore, I would think Ruby would be too afraid to even approach Chomper, given she would assume he was trying to eat her if she saw him. What got her to trust him?