So I'm trying to make a homebrew styled from the flying nimbus from dbz, but no matter what I do, it never comes up as a separate item. Instead of it just saying "add", it gives me the option for equipment, backpack, ect. Can anyone please help me with this?

Can you provide some more details as to what you're trying to do - are you trying to create a homebrew magic item or add a custom object to a character sheet?

Typically, mounts are creatures and located under Extras -> Mounts on the Character Sheet. If you're just looking for a flying mount, you could add something like a Giant Eagle and then change the name, size, type, AC, etc. If you want to add special abilities to it or use it across multiple characters, then a homebrew monster would probably be the way to go, though you could make it a magic item, if desired.

A magic Item, like a wonderous item

The animals in the that can be added u see Equipment are structured as “containers.” We can’t make those as homebrew.

ah, that sucks.

IIRC the flying nimbus could move on its own accord and seemed to have a personality, so maybe you flavor this as some sort of wind elemental homebrew monster? I did something similar with constructs to make motorcycles for a modern campaign. If you do it that way, then you could benefit from something like the Mounted Combatant feat. Alternatively, you could use Carpet of Flying as the basis for the item you're looking for.