If I use Koraidon's "Unrelenting Onslaught" attack, can I get the extra bonus damage if I attacked last turn with a different Koraidon?
Yes. Unrelenting Onslaught only cares about whether a different Ancient Pokemon attacked during your last turn.
Source: Surging Sparks FAQ; TPCi Rules Team (2024-11-07)

Relevant cards

Note: All applicable cards may not be listed below.
SV8_116
SV8_116