There are two unused pieces of headgear in the files of the original Nintendogs games: a chonmage wig (a Japanese male ponytail haircut worn by samurais in the Edo period to hold their helmets in place), and an afro wig. The chonmage has an unused description found in the Japanese version of the games and had all of its assets in the international releases replaced with the data for the Rainbow Wig. The afro on the other hand was finished enough that you can use cheat codes to make a dog wear it, but it does not have any leftover text or item attributes that let it be obtained as an item. It may have been cut from the games late into development due to concerns over racial offense and replaced with the Rainbow Wig, which has a very similar appearance to the afro, but with a rainbow-colored texture instead of one resembling black hair.

The Papillon and Maltese breeds were in development for the game but never finished. Their placeholder files can still be found in the game's code. The Maltese did eventually become a selectable breed in Nintendogs + Cats.

The Boxer, Siberian Husky, Golden Retriever, and Dalmatian breeds are not in the original Japanese versions of the game. They were added in the localized versions. These breeds would later be included in Japanese regions for the first time in Nintendogs + Cats.

A Wii version was considered, but it was ultimately decided that the voice recognition, utilizing the DS's integrated microphone, was too integral to the experience. Because the Wii did not have a microphone, the player's dog would not be able to respond to their voice.

It was apparently decided that it would not merit the creation of an external peripheral, such as the Nintendo 64's Voice Recognition Unit required for Hey You, Pikachu!, despite the creation of the Wii Speak accessory alongside Animal Crossing: City Folk in 2008.

During development, Nintendo had considered other animals for the game that became Nintendogs, as the preferences of the development team varied, with the main programmer being "more of a cat person". Shigeru Miyamoto's decision was influenced by his own experiences with the family dog, a Shetland Sheepdog named Pikku: