Online RPG sidescroller, requesting tips

Hello guys, I am decent java coder, and I probably can write anything. There is a game I’ve been working on for a year and managed to create own java game engine that currently contains advanced map editor. And the game it’s self (client and the server) Right now people can join and see eachother and jump together and chat. Also server sends the stats of the player like level, health etc and applies it to the client. Everything works perfectly.
So, I am asking for some tips and techniques on making mob system, (I have the mob engine in the client, it has same physics as the character but with the different speed and it walks and jumps randomly and jumps if he sees that there is another platform next to it) so what should I do to make the mob online? Should I create another server specifically for mobs? Or should I add it to the main server? will it add unwanted lag? what should I do?

(this is how it looks like [made this video loong time ago])

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