https://pastebin.com/D9MK9f3fI really doubt that they started the 2nd movie much before the 1st movie was released (july 98 I think)
so that would make Lugia's design as being from the third quarter 1998
Also Lugia isn't in the May 1998 sprite dump
there's several pokemon that Lugia might've taken inspiration from, but Lugia itself isn't in.