Looking on the internet deeply has found these results:

this is a dog is the most popular phrase on the web.

but this is a cat 112,000 results on the web	this is a dog 132,900,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Nov 10, 2014 ... But this is a cat that, unlike every other cat I've met, tidies up after himself. Also, clearly, a dreadful warning to mothers who don't stay home all�... Dec 8, 2014 ... One of our kitties vanished for a week last October�they're both indoor/outdoor so we expect irregularities, but this is a cat that has never�... I'd say in addition to having a problem with the cat you have a problem with your BF, trust issues for starters but this is a cat thread so I'll stick to�... Apr 3, 2014 ... People might call me crazy, but this is a cat site, and I expect that a lot of you understand how we feel about our cats. If I have to give something�...	I mean, this case is a dog. This is teaching a dog to become a butterfly. Look, Auntie Ayse, this is a dog. This is not a cat. This is a dog. This is actually a dog couch, but it's super comfortable. Do you think this is a dog, dog? This is a dog, Sol. This is a dog resembling a horse.

but this is a cat

112,000 results on the web

this is a dog

132,900,000 results on the web

More popular!

Nov 10, 2014 ... But this is a cat that, unlike every other cat I've met, tidies up after himself. Also, clearly, a dreadful warning to mothers who don't stay home all�...
Dec 8, 2014 ... One of our kitties vanished for a week last October�they're both indoor/outdoor so we expect irregularities, but this is a cat that has never�...
I'd say in addition to having a problem with the cat you have a problem with your BF, trust issues for starters but this is a cat thread so I'll stick to�...
Apr 3, 2014 ... People might call me crazy, but this is a cat site, and I expect that a lot of you understand how we feel about our cats. If I have to give something�...