anyone know how to make an item that spawns custom mobs

It's simple, I'll tell you in steps:

1- create an item

2- go to the triggers section

3- create a procedure in the "when right-clicked" section

4- Once inside, go to the "World management" section and look for the procedural block called "spawn entity..."

5- choose the entity you want

6- enjoy your item that summons creatures :D

Its really easy

1 make the item you want

2 create a trigger and make a procedure whenRightClicked

3 Attach spawn entity (the entity) 

4 Attach remove (you're custom item) from main inventory

for a final touch you can disable the spawn egg