Why is イチゴ written in katakana?

Question

From which language did it come to Japanese? And why does it have its own kanji?

Answer 1

いちご is a native Japanese word, which is almost as old as the written records of Japanese itself. (Apparently it first appears as イチビコ in the 日本書記{にほんしょき} (8th century) and as イチゴ two centuries later in the 倭名類聚抄{わみょうるいじゅしょう}.)

As practically all native Japanese words (like 雨{あめ} rain, 村{むら} village, etc.), いちご, too, was assigned a corresponding kanji from Chinese, in this case 苺.

Fruits (and more generally, plants and animals) are often written in katakana, so you shouldn't be surprised to see it written as イチゴ. (See for example this question.)