How can I make an entity generate at height Y = 125?

That will spawn your entity if you cant see it and isnt near you. But i dont think its good. And i forgot how own entitys work so sry if this didnt help you.

Did it work?

You can make the mobs teleport to y = 125 whenever it spawns.

So basically, your mob spawns on the ground or something. But if you go to entity procedures, you'll see you can make a procedure for when mob spawned. There, you can make a new procedure that just teleports it up to y=125. If you don't know how, just do this:

Set location of event/target entity to    x       125        z

And I think that's it.

@OP here: 
Trigger: NONE (As this is called by your living entity)

IF "Can location at XX YY ZZ see the Sky = TRUE"
v
Do "Spawn XX YY ZZ Pitch vx vy vz living Entity: CustomEntityhere"
Return = True
v
Return = False

THEN save it then on your living entity: check everybox (spawning/despawning) then adjust the other values AND on the lower part, select your new spawning procedure method. This will replace the "Creature/monster/sea creature/ambient" spawning rule.

"Can location at XX YY ZZ see the Sky = TRUE" checks if the block or area is open, (making it sure that it's not underground or something.

"Spawn XX YY ZZ Pitch vx vy vz living Entity: CustomEntityhere" Is a better choice instead of the standard "spawn XX YY ZZ LivingEntity" as the VX VY VZ manipulates area where your mob is gonna be placed.

Return = True basically keeps this whole procedure and will try to spawn your entity on areas that can see the sky (no overhheads)

Return = False is a stopper when an area have an overhead, let say a big mountain.

But remember, this script is called by your LivingEntity so spawn weights should still matter. (Make sure to set it like 10 to 20, (Passive mobs have 10 and lower like pigs/sheeps)