"a" is a singular term. "a pine cone", "a banana", and "a piano" are also singular terms. Therefore, "a red howler" connotes one red howler. I'm guessing a "pack" of something is plural. "A pack of cigarettes", a "pack of wolves"= plural. Therefore, logically, one of something... a red howler is not a pack.
Now, if you had a bunch of red howlers (plural) instead of "a" red howler (singular)....they could probably be considered a pack.