I read a fanfic on FimFiction that I can't find.
Dragons resist magic, and that gets stronger as they age.
And because of that, Twilight uses Spike as a test subject for her spells because they can't harm him, and as a strength measurement. If they can affect him, then her spells are strong.
One example of such a spell is the one that gives him a temporary moustache / mustache. Not sure which spelling the story used.
Quite sure it's not Entry 7, Part 1 (Boast Busted) - The Poisoned Barb's Tale - Fimfiction...
Or maybe it is? Somehow I read the first few chapters, which I totally forgot about? But is there any with a male Spike??