Ringwarden Owl — ruling 1

Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.

Ringwarden Owl — ruling 2

Any spell you cast that doesn’t have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won’t cause prowess to trigger. Playing a land also won’t cause prowess to trigger.

Ringwarden Owl — ruling 3

Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.

Ringwarden Owl : règles, fonctionnement et interactions (MTG)

Ringwarden Owl is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying (This creature can't be blocked except by creatures with flying or reach.) Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

How it works : Ringwarden Owl

Original English text — Wizards of the Coast / Scryfall

#1June 22, 2015· Wizards of the Coast
Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.
No other card is named in this ruling.
#2June 22, 2015· Wizards of the Coast
Any spell you cast that doesn’t have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won’t cause prowess to trigger. Playing a land also won’t cause prowess to trigger.
No other card is named in this ruling.
#3June 22, 2015· Wizards of the Coast
Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.
No other card is named in this ruling.