Why does Takeshi Yamamoto have a interesting naming sense? In Katekyo Hitman Reborn, Haru mentions that Takeshi has a interesting sense of naming. 

Now I were wondering, is this only because he added the 'Ko' character to the name? Or is there a deeper, under-laying joke/reference made there?
Jirou is an Akita Inu, while Kojirou is a swallow. They are named similarly because they are two halves of Yamamoto's Vongola Box.

Kojirou is named after the swordsman Sasaki Kojirou, known as the rival of Miyamoto Musashi and for his "Tsubame Gaeshi" (Turning Swallow Cut) technique that is said to mimic the motion of a swallow's tail. 

Jirou is probably named after one of the two Sakhalin Husky sled dogs that survived an ill-fated Japanese Polar expedition. While the breed is currently extinct, they are said to be one of the precursors to the modern Akita Inu. 

These two animal were not only named after figures of historical origin, but also share similarities to one another as they are both two parts of a whole. The "ko" in Kojirou means "small/little."