The difference is that Charlotte is a foreigner, and that where she comes from, Ravens are popular familiars. Ferrets on the other hand aren't really seen or mentioned at all from what I know (outside of a single mentioning of charmed ferrets in one of the books.)

[quote=Yog Sothoth]
Albus had a ferret when he went to Hogwarts
[/quote]

I don't remember this, can you remember where you're getting this information? That was quite a long time ago, and regulations may have very well changed since then.