nidan verb (upper class) with 'ru' ending (archaic), yodan verb with 'ru' ending (archaic), nidan verb (lower class) with 'ru' ending (archaic), intransitive verb
Archaic verb form; in modern Japanese, 恐れる is used instead. Found in classical literature and fixed expressions like 恐るべき (fearful, formidable). Multiple archaic conjugation patterns (Nidan/Yodan) and marks it are listed as intransitive.
In classical texts, it was used like 'to fear the enemy'.
The form 恐るべき is still used today, as in 'formidable talent'.
恐れる is the modern standard verb for 'to fear', while 恐る is its archaic form found mainly in classical texts and fixed expressions like 恐るべき.
怖い is a common modern adjective meaning 'scary/frightening', whereas 恐る is an archaic verb meaning 'to fear', so they differ in both era and part of speech.
Archaic verb form; the modern equivalent is 恐れる. This spelling is the most common kanji representation in classical texts.
Archaic variant kanji, emphasizing fright or terror.
Rare archaic variant kanji, with a nuance of dread or apprehension.
Archaic variant kanji, often with a nuance of awe or reverence rather than simple fear.
Archaic verb; the modern form 恐れる developed from it. The exact historical derivation is uncertain, but it is a native Japanese word.