Petrichor is a small datapack focused on giving new mechanics to the Sniffer and its plants. Most content fits with the vanilla game, and does not interfere with it. All the content can be discovered in-game through advancements and other clues.
YouTube showcase: https://youtu.be/piDY2Z5ib0E
Make sure you download and enable the required resource pack to play with this data pack, you can find it by clicking in the version you want to download and scrolling down a bit. This isn't needed if you are using the mod version, as the resource pack is bundled with it.
Datapacks are not able to add custom items or spawn eggs to the creative inventory. The following works for a lot of datapacks, including this one:
/loot give @s loot <datapack>:items/<item>
/function <datapack>:commands/summon/<mob>