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Mutt
Informal Usage
The term 'mutt' is informal and is used more in casual conversation than in formal writing. 
Not Derogatory
'Mutt' can be endearing and is not generally intended as an insult when describing a dog. 
Neutral Connotation
The word 'mutt' is neutral, but the tone of voice can change its connotation from affectionate to dismissive. 