Pet alone evolves as larger animal (8)

I believe the answer is:

antelope

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

'animal' is the definition.
(I've seen this before)

'pet alone evolves as larger' is the wordplay.
'pet alone' can be anagrammed to 'ANTELOPE'.
However, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for antelope that I've seen before include "A graceful gazelle from Africa" , "Wild creature" , "One petal for this animal" , "horny beast!" , "Hoofed beast" .)

I've seen this clue in The Sun.

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