Try this in B1:

=IF(A1="Dog",E21,IF(A1="Cat",F21,IF(A1="Mouse",G21 ,"Not specified")))

The phrase "Not specified" is returned if none of the criteria are met.
There is a limit of 7 to the number of IF statements you can "nest" in
this way - if you have more criteria then there are other ways of
achieving what you require.

Hope this helps.

Pete