According to the OED, in the US 'scallion' can also mean what we call leeks and shallots as well as spring onions. It can also mean, not just in the US apparently, 'Welsh onion' (whatever that is).
Apparently the origin of the word is '[onion from] Ascalon' (in what is now Israel). It is first found in English in the fourteenth century:
'For thy lyff and thy barouns
He wyl not geve two skalouns.'