XF 1.5 Example / how-to for using <xen:selectunit... selection to show/hide extra fields

[Yugensoft]

In my admin form, when I click to select a new list item in the selectunit, I want to change the fields below that unit. E.g. animal -> select dog; new field 'breed' appears.

I'm just checking there isn't some built-in Xenforo way to do this, before I do it with standard js.