According to this article also posted in the page comments, the dog in the first series was named Pharaoh played by a male dog named Roly. The name of the dog was never mentioned on screen in the first season, however Robert does refer to him as "boy" in Episode 1.01. The article also states that the offscrean story is that Pharaoh died sometime between the end of series 1 (1914) and the start of series 2 (1916).
From the second season on it has been Isis, played by a female dog named Ellie. The reason for this post is to ask whether a second page get created for Pharaoh, or just rename the page to "Pharaoh and Isis" and cover the history of both dogs on a single page?