I want to get onto a Pixelmon server, but I'm wondering if I need the mod installed locally in order to be on the server?


Indeed you do.

Some mods do not need the Server to have it also installed, and vice versa.
IE: Optifine installed on client doesnt require it to be on the server.

But any mod that adds a new item/block for everyone to use will need to be installed on both your Minecraft and the server.

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