Plotocol Support made it possible to connect with 1.12 even with 1.13 server, but the plugin did not work with the following error.

So they see theirselves as the entity but nobody else does. Removing the moreplayermodels.entities permission from players still allows them to turn into entities on their client, but it wont be updated to players anymore. To prevent people from seeing 'secret chats' in chatbubbles, set the EnableChatBubbles to false in the config file.

moreplayermodels.entities (remove this permission to disable players from turning into other entities).

/mpm sendmodel (also works on citizens2).

/crawl to crawl (need to hold shift to crawl around).

This however will only be visible to players who have the MorePlayerModels mod installed. The only thing you need todo is press X or type /sit to sit down or press Z or type /sleep to lie down. This plugin allows you to sit and lie down whenever and wherever you want. Without moreplayermodels you wont see eachother sit, sleep, dance, hug or love or any of the model changes This plugin requires Players to install the MorePlayerModels mod

More player models 1.12.1