In my case, a duck and a horse; also a cat (5)
I believe the answer is:
moggy
'a cat' is the definition.
(informal term for a cat)
'in my case a duck and a horse' is the wordplay.
'in' is an insertion indicator.
'case a duck' becomes 'o' (I can't explain this - if you can you should believe this answer much more).
'and' means one lot of letters go next to another.
'a horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'o'+'gg'='ogg'
'my' placed around 'ogg' is 'MOGGY'.
'also' is the link.
Can you help me to learn more?
(Other definitions for moggy that I've seen before include "Puss" , "Pussy" , "Cat (sl.)" .)