Pretty sure if that big adorable lug wasn't in all the episodes he certainly was in most of them.
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You have many question, Mr Sparkle. I send you premium -- answer question, hundred percent!
Animals generally do not get on screen credit. If you look closely, most of his credits that are listed for the show clearly say he was uncredited. That's why he's only shown as being in a handful of episodes. He's not an official cast member.
Plus, the information on IMDB is user generated. They don't have staff trolling old tv shows making sure actors - let alone animals - are being added to credits. Especially when they were not credited in the first place.
If you're so upset about it, why aren't you adding him yourself? Granted, users shouldn't be adding cast members unless they're absolutely sure they were in an episode. Don't just willy nilly add him to all the episodes simply based on guesswork.
The people you idolize wouldn't like you.
Perhaps those are the only episodes with speaking parts? ;)
/ joking, but now that I think about it, I don't remember that many episodes where he barked