I saw a Japanese rhinoceros beetle fighting with a stag beetle.
I saw a Japanese rhinoceros beetle fighting a stag beetle.
"カブトムシとクワガタが戦うのを見た"
カブトムシ a Japanese rhinoceros beetle, a Japanese horned beetle
AとBが戦う A fights B, A fights with B
見る see 感覚動詞の使い方 see A ~(原形不定詞)あるいは現在分詞
例: I saw her stand in front of the station. I saw her standing in front of the station.
I saw a Japanese rhinoceros beetle fighting with a stag beetle.
I saw a Japanese rhinoceros beetle fighting a stag beetle.