Standard word for the animal; no special nuance.
A donkey is carrying luggage.
The boy rode a donkey along the mountain path.
They keep a donkey on their farm.
ラバ is a mule, the hybrid offspring of a donkey and a horse, whereas ロバ is the donkey itself.
Katakana spelling is standard; the word is usually written in kana.
Kanji form is rare and mostly literary or historical.
Hiragana spelling, also common.
A Sino-Japanese word from Chinese 驢馬, literally “donkey” or “ass.”