The abandoned puppies are minor characters of Ginga Densetsu Weed in the Hōgen Arc.
They were left in the mountains (by who however is unknown). Then an English setter, named GB, finds them and brings them back to Nero for protection. However, Hōgen went to check on Nero that same day. Hōgen thought Nero was hiding a female from him, so in order to prove that the puppies weren't his, Nero had to kill both of them.