yellow bittern
A small heron species (Ixobrychus sinensis) found in East Asia. The Japanese name is usually written in kana. The kanji 煩悩鷺 is a playful or folk-etymological spelling.
See also: よしごい
I spotted a yellow bittern in the rice paddy.
Yellow bitterns often hide in reed thickets.
The reading ぼんのうさぎ is a jukujikun (special reading) for the kanji 煩悩鷺. The characters 煩悩 (worldly desires) and 鷺 (heron) are likely a playful or folk-etymological combination, but the exact origin is uncertain.