You have already defined the ELSE part in your expression and it is returning DB as default. To have another value as default, you must have at least one more condition, like:
You have now :
If ( XX='Cow', 'Animal', 'Not-animal')
To have default third value, you would need something like:
If( XX='Cow', 'Animal', If( XX='Tree', 'Tree', 'Neither animal nor tree'))