I've tried to use Vlookup for this but was unable to get it working.
Here's what I need:

In column 3 I have:

dog
dog1
cat
cat2

I'm trying to get an inventory management tool by looking at column 3 and
subtracting 1 from the previous row, so I would have:

Column 3 Column 4 for dog Column 5 for dog1...
100
dog 99
dog1 0
cat 0
cat2 0
dog 98

So, if I have "dog" in row 3 then I get a formula to look up "dog" then
subtract 1 from the previous row (or previous valid number, maybe not 0) and
give me the result, and if it is not "dog" it'd show a 0.

Hope I was clear enough... PLEASE HELP!