"Terrible" has two meanings though -- invoking terror and being crap. Ivan the Terrible wasn't a crap Czar -- he just was somebody you didn't want to get on the wrong side of.
In modernity it has two meanings, but its roots are tied to terror. It's only been adapted as a colloquialism to mean "crap". Somewhat like how "aweful" and "awesome" have drifted to mean totally opposite things when their suffixes and etymology suggests they should and originally did mean something very similar.
Does something like a diplodocus genuinely strike terror into you?
They probably looked quite cute -- but so do hippopotamuses, which despite being herbivores kill more humans than do lions. Never underestimate a large animal. Even cows can be deadly.
A hippopotamus has a jaw that will break a watermelon - don't mistake them for cute!