The direct translation would be "Eso".
Therefore your senteces are to be written like this:
Eso es bueno.
Eso es un perro.
Although if I must admit, it's not often used in such way, but rather you'd hear/read them as:
Es un perro.
Which -even though I ignore the actual reason, and therefore can't faithfully explain it to you- would be some manner of "abbreviation", just as when instead of writing "It is" the "It's" appears:
It's a dog.
It is definitely not the same mechanism, but rather just an analogy the one I'm using.
Hope it helps!.