So I wanted to make something similar to a Drow Mage - but change up some of the spells, not all of them, for a specific Drow to be encountered.

I went into the Homebrew, Create - and selected ALL - but in the drop down the only DROW listed is just... Drow. No Drow Mage. While that's not that big of a deal, coding all those spells can be a bit cumbersome, especially if you only wanted to swap out one or two of the spells, make some slight changes to AC, HP, etc.

A nice feature to have is if there's an existing Monster have a "EDIT - Make Copy" directly from that monster's page. And what that would do is literally the same thing as the "Create - All" - except it'd be on ANY monster you have access to. This would allow me to click "Edit - Make Copy" on the Drow Mage, if I happen to be searching through and say, "Ah ha! This is close! Just need to make a few changes!" 

I just checked and can make a homebrewed monster based off of the Drow Mage, as well as Drow Elite Warrior, and Drow Priestess of Lolth.

We can only make homebrews based off of official content that we have unlocked access to through purchase. Only the basic Drow is available as free access under the Basic Rules & SRD. To be able to use the Drow Mage, you would need to purchase either the Monster Manual, or at least that stat block. If you can read the stat block, it must be shared with you in a campaign by someone who has purchased it.

Quote from IamSposta >>

I just checked and can make a homebrewed monster based off of the Drow Mage, as well as Drow Elite Warrior, and Drow Priestess of Lolth.

We can only make homebrews based off of official content that we have unlocked access to through purchase. Only the basic Drow is available as free access under the Basic Rules & SRD. To be able to use the Drow Mage, you would need to purchase either the Monster Manual, or at least that stat block. If you can read the stat block, it must be shared with you in a campaign by someone who has purchased it.

 
Ah ha. I bet it's because it's shared. I hadn't even thought of that.

Thank you, yet again! :)

As usual, happy to help.

I just spent well over an hour creating and publishing a homebrew monster for my campaign along the Misery River - Huge Plesiosaur. How do I copy that and modify it into Gargantuan Plesiosaur? Do I have to start over?

Got advice on another thread.  I don't know which of these steps is optional, but this worked -

1. Published the monster

2. it was accepted

3. Navigated to Collections | My Homebrew Creations | CREATE A ... | Monster

4. Left MONSTER TYPE as All, and entered MONSTER = Huge Plesiosaur

and, it worked!

Yes, you can use your own homebrews as templates.