Tonks can honestly say that they wouldn't have seen the dog if it weren't for the roots in the garden. Every morning they wake up first and go outside and vanish the milk that Charlie has left out for the brownies. Every. Single. Bloody. Morning.
Do you want rats Charles? Because this is how you get rats.
Anyway using their incredible auror skills they once again trip over the roots in the garden. Every morning. And from their new vantage point (the ground) they see the tail; thick and bushy and fairly incongrous considering that their one rule upon agreeing to share a house with Charlie was NO PETS!
So, grumbling about the inconsiderate bloody gobshite they consider their best friend they go to poke the no-doubt dangerous animal with their wand and discover…
An ordinary dog.
That's definitely not Charlie's style.