Single-encounter Pokemon are missing

In Generation IV, diamond pearl and platinum, many legendaries can be encountered exactly once. Some appear in static locations, while others will roam across the map once encountered. A third category exists for Spirtomb's bizarre encounter procedure.

Currently, the API does not include any encounter entries for any of these three methods. Dialga, Palkia and the (unreleased, event-only) Arceus have no data. This seems inconsistent with Generation III, which lists Mew's event-only encounter: https://pokeapi.co/api/v2/pokemon/151/encounters

My current project uses this API to calculate which games are required for which Pokemon. If this feature were not added, then I would have to manually add legendaries.

Start by inspecting the API encounter records for Dialga, Palkia, and Arceus, then compare them with the Mew encounter endpoint linked in the issue. Identify where Generation IV encounter data is stored and how static, roaming, and Spiritomb's Hallowed Tower encounters are represented. Done means the missing single-encounter methods are available through the API.