恐ろしい includes a sense of awe. Also, 怖い is subjective, and 恐ろしい is more objective, my dictionary says. Back to your example, you can use whichever, but 怖い怪物 sounds that the speaker himself knows about the monster very well through his experience or some sort. Colloquially 恐ろしい怪物 is more common.

恐ろしい includes a sense of awe. Also, 怖い is subjective, and 恐ろしい is more objective, my dictionary says. Back to your example, you can use whichever, but 怖い怪物 sounds that the speaker himself knows ABOUT the monster very well through his experience or some sort, so colloquially 恐ろしい怪物 is more common.

んん、やっぱりそうだね。例えば話をする時、”主人子が「怖い・恐ろしい」怪物と戦った。”この状況にはどちがいい？

そのときによってどっちをつかうか変わるりますよ＾＾ どういう状況で使いますか？