Hi! This is for anyone who wants to make an unmodded custom mob. Note that this can only be done in Creative Mode.

Press T and type: /give @p command_block
Press T and type: /gamerule doCommandBlockOutput false
Place the block.

Go to minecraft.tools and select Mobs in the left bar.
Select the mob that you want the drops to be like, select one like endermite for no drops. Also choose for roughly the AI.
Give it a custom name. Actually name it the official thing.
Give it a custom head, a block for a "blockhead", or a helmet for a floating head.
If your mob is humanoid, possibly give it blocks for paws (in hands), and armor.
You can choose for options, for example:
Glowing for you to know where it is. Useful for pets.
Invulnerable, Silent, Persistent, NoGravity... well, you get the idea.
Skygliding With Elytra for if you want your mob to fly after falling. Note that this doesn't mean flying always.
You can also give it potion effects.
Give the mob invisibility (But not if you want to show the vanilla mob).
Make the mob ride something (don't worry, that'll be deleted)
You're done with the main command, copy it from below. Paste it into your in-game command block.
Don't spawn it yet! It doesn't have enough features.
You can use preset powers, like guardian lasers. Find the Passengers Tag.
Type in the "id" tag, well, an ID. Like I said, guardian for lasers and such. Be sure to use lowercase!
Type after that, still in quotations, CustomName:"[][]"
Replace [][] with a testing name.
Place a repeating command block. Set it to always active.
/effect @e[type=[][],name=[][]] invisibility 20
Replace the first [][] with the ID you used, the next [][] with the testing name.
If you want more than this (you should), place more "Always Active" repeating command blocks.
Alternatively, you could use scoreboards to make actions once every second or minute or whatever.
/execute is really helpful. You can give the mob particles, damaging effects, & more. Have fun!

_mouse36