怖い is for more in-your-face "terror", while
恐い is a more abstract notation of "fear", or "dread".
怖い is used more for something seen or experienced first-hand, while
恐い is used for things one may never actually encounter first-hand. Compare the following:
- 地震は恐い → "Earthquakes are scary"; a general concept - you don't have to have actually experienced one to believe this
- 地震は怖かった → "The earthquake was scary"; you were in this earthquake - it gave you personal, scary/terrifying feelings
恐らく, it means "regrettable" in the sense of "I'm afraid that..."
I'm afraid I don't know the answer. = Regrettably, I don't know the answer.
In this day and age, "not knowing the answer" to something does not actually make you scared or fearful. Although I can see how this modern usage might have carried on from older times, when a servant had to tell regrettable news to a master/king/etc., and might actually have feared receiving some punishment for delivering bad news.
恐らく is really more translated as "perhaps" or "probably", but you can see the overlap in the following sentence:
恐らくそれは見つからないだろう → I am afraid it will not be found. / It is not likely to be found.