'X' can mean zero.
EDIT:
Grond, HOTU is a good example.
I don't think so, I remember playing in a Canadian tournament when was said that Grond could be used with X meaning 0, yes, that I think is right, but also remember they saying that Grond was an exception, and that was why it could have a X meaning 0. It is a little confuse, but i think if it is an exception, maybe other cards can't have their X meaning 0. Not really sure about the Stamp Savage minion, but maybe he can't have his X meaning 0.
Hope I have helped.