katakana is used for words brought in from other languages. presumably animals that the Japanese have long been familiar with have native terms, that thus get written in kanji or hiragana. other animals that were integrated in the language by foreign influence get katakana. I infer from the spelling in hiragana that the Japanese have long known dolphins.

what caught me by surprise was learning that ramen is spelled with katakana. I'd have guessed it was "native" from Japan. but that would be a mistake. the word is borrowed from Mandarin, which suggests that the dish originated in China.