Gere gets wild with horny beast (4,5)

I believe the answer is:

grey heron

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

'gere gets wild with horny' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'beast' is the wordplay.
I cannot quite see how this works, but
'beast' could be 'heron' (heron is a kind of beast) and 'heron' is present in the answer.
The remaining letters 'grey' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Independent.

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