The animals went in two by two

Why is it that so many words for female animals are used in a derogatory way when applied to women, but the equivalent terms for male animals are often used in an admiring fashion? I mean, when someone wants to be really offensive to a woman she is called a bitch, a silly cow, a stupid mare or a spiteful vixen.

What impression do the terms 'stud' or 'buck' convey? Virility, power, sexiness. Not sweat, stench, hairiness.

But why are some female terms universally disrespectful while others, although screamingly patronising, aren't intended as being so? 'Pen' for example. Or 'duck' (eek!) or 'hen' (considered perfectly okay north of the border). Another thought: in the 1960s 'bird' or 'chick' was an eminently acceptable appellation for a man's female companion. Now it's likely to get a chap thumped!