I find the sentence a bit strange indeed. I would never refer to a cast as "isto" because it's a word mainly used for objects, just like I wouldn't use it to refer to a person. So unless I was referring to an object shaped like a cat, I can't see this happening.
But, on the other hand, "este" is used mainly for people, so it doesn't sound quite right either when referring to an animal. (But you could say "Este é o Tareco", referring to a cat you do know, kind of like a person). I'd say it's more likely to drop the subject and simply say: "É um gato". But then again, I don't recall a situation where one needs to point out that a cat is, in fact, a cat. That can explain why all possible sentences sound so strange; because they're not very used.
Also, to add to Farinhas94's comment, you can think of «isto» as simply "this" and «este» as "this one." Also, «isto» can refer to anything without having to change with gender; it is neutral that way, but it can only be used for singular objects. However, «este» must agree accordingly, with gender and number: «esta», «estes», and «estas».