Tadakichi-san (忠吉さん), also known as "Mr. Tadakichi", is Chiyo's pet dog. A mild-mannered and gentle Great Pyrenees, he is large enough for Chiyo to ride on his back and indulgent enough to allow her to. Sakaki particularly likes him, because he allows her to pet him without biting her, something Kamineko has done at most. Chiyo describes him as a gentleman. Sakaki also daydreams about being able to ride Tadakichi as Chiyo does.
Chiyo takes him for a walk every evening and after Sakaki and Chiyo become friends, Sakaki often joins them. The fact that Pyrenees come from France is something Osaka has referred to more than once. When Maya finds Sakaki after travelling all the way from Okinawa, Chiyo suggests that he should stay at her house until Sakaki gets her own apartment (since Sakaki's mother is allergic). Chiyo, along with her friends, was a bit worried about how well Maya and Tadakichi-san would get along, but for nothing, seeing as they respected one another.
When Chiyo moves to America after high school graduation, Tadakichi-san comes with her, staying at the house of some of the family's friends.