Donphan only learns Curse via a TM in gen 2. Gen 2 pokemon cannot be transferred to generation 5, thus making it impossible for Donphan to learn Curse in gen 5, or any gen other than 2 or 7 (via virtual console transfer).
Additionally, this is not the place for bug reports. Bug reports have their
own thread! But always remember to ask in the
Help room on pokemon showdown before posting in the bug reports.