Pixelmon Mod

A quick idea - when the Player catches a Pokemon that they haven't owned before, a pop-up of that Pokemon's pokedex entry could appear and the black outline/empty entry be filled in, similar to the original games.

To add a level of smooth implementation, perhaps it could be displayed as a toast (what advancements and recipes are, in the top right), perhaps with a red background, as most Pokédexes have red on them, including the one in the mod, rather than a full UI popping up before or after the items prompt

I prefer dhi's version. Gives a notification while not disrupting the game for people who aren't too interested in it.

You could probably map it so that if you open the Pokédex while the toast is active, it opens on that Pokémon's entry. That way you can choose to look at the new entry immediately - or not.

This is a good qol suggestion especially since it fits to what the main series does.