Quick Create Forms

From CRM 2015, you can use Quick Create form to create records quickly. 

To enable Quick create form for any customize entity, we need to update entity. 

 1. Check "Allow Quick Create" in entity customization. 

2. Save Entity and Publish. 

Now if you try to create new record from lookup it will open quick create form. 

Some times when you enabled Quick create form from entity, and haven't created Quick Create Form, then system will open normal form instead of quick create. 

Also If you have Quick Create form but Allow quick create is not checked, then system will open normal form instead of quick create. 

In another words, to use quick create form, entity must be checked for "Allow Quick Create" and at least one Quick create form created and Entity changes are published. 

If you created multiple Quick Create forms for any entity, one first created Quick create form is used when you are using quick create functionality either from lookup or sub grid. 
There is no Option to choose another quick create form, also you cannot disable Quick create form. 
In case if you want to use 2nd form only then you have to remove first created quick create form and then system will automatically pick second quick create form.