noun
Refers to the waterfowl. The kanji 鴨 is standard, but kana is also common.
公園の池に鴨がたくさんいる。
There are many ducks on the pond in the park.
noun
easy mark; sucker; sitting duck
Figurative use: a person who is easily taken advantage of, cheated, or defeated. Usually written in kana (かも).
彼は詐欺師にとって格好のかもだ。
He is a perfect sucker for a con artist.
あんなに無防備じゃ、まるでかもだね。
Being so defenseless, he's like a sitting duck.
Rarely used kanji form for 'duck'.
アヒル refers specifically to the domestic duck, while 鴨 is a general term for wild ducks and the waterfowl family.
The origin is uncertain, but the word is native Japanese. The figurative sense likely derives from the idea of a duck being an easy target for hunters.