The ID of the foal is 96 and the "tame horse" (which doesn't dump you) is 86:
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//Make adult
	if (gotstate("self","tame")) {
		play "build_finish.wav"
		local $x,$z;
		$x=getx("self");
		$z=getz("self");
		create "unit",86,$x,$z;
		freevar $tmp,$x,$z;
		free "self";
	}
script=end
Any suggestions? It's probably something very simple

EDIT: Works if you put it under on:use, but it'd be nice for it to happen automatically.
I messed up another script.
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Offline
create "unit",86,$x,$z; (WRONG)
$id=create("unit",86,$x,$z); (RIGHT)
Apache uwu