Bull or cow? Horse close (4,4)

I believe the answer is:

grey seal

I'm a little stuck... Click here to teach me more about this clue!

'bull or cow?' is the definition.
I can't tell whether this definition defines the answer.

'horse close' is the wordplay.
'horse' becomes 'grey' (I've seen this before).
'close' becomes 'seal' (sealing is a kind of closing).
'grey'+'seal'='GREY SEAL'

Can you help me to learn more?

(Other definitions for grey seal that I've seen before include "N. Atlantic mammal" , "Sea creature" , "Marine mammal" , "North Atlantic mammal" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.