Hello everyone!
I have been Spigot for years and years and know a crap ton about it. How to develop plugins, how to run/edit things.
And I’ve been developing a MMORPG inside my spigot server. I’ve came to realize that it would be AMAZING to have actual new mobs like Goblins, Warlocks, etc.
With sponge, can I somehow add custom new mobs and have it perfectly synced through all the clients? And if so, how can I do this?
Please let me know with the most information you can, thanks!
Short answer: No.
Sponge is limited by the same problems all other modded servers are - without a modded client, there’s no way to do more advanced stuff like new mobs/blocks/etc. However, SpongeAPI has been designed to be as mod-friendly as possible, so that you can interface with mods (unlike, for example, Bukkit).
Well that is unfortunate 
I thought sponge did have a modded client to sync everything with it, but I guess not.
Thank you for your answer.
Sponge facilitates working with Forge mods in the context that you’d be able to add entities through Forge, but still work with SpongeAPI as a plugin to handle everything else (like damage, item creation etc.). This way, you can do a huge amount of customization from both worlds.
If you meant like sending packets to the modded clients using a Sponge plugin? You will need to use Forge for that since Sponge does not allow sending any kind of packets.
As gabizou pointed out:
You do everything except mob/item creation in Sponge. The modded items/mods are then created through Forge.
So Sponge somewhat has a “modded” client which is based on Forge. You can run Forge+SpongeForge on the client right now. Just install Forge on the client as usual and drop Sponge into the mods folder (same for the server).
This gives you maximum compatibility between versions (as Sponge tries not to break) and reduces relying on Forge to a minimum.