What are some less obvious creatures that a powerful wizard might keep in their tower, either as a guardian or servant? Looking for some inspiration for my setting.

Invisible stalkers? 

For my setting, every mage and archmage has a homunculus. Xorn is an interesting pick: many spells use gems as consumed components. Xorns are basically walking gem detectors, as long as you can train it to not eat the gems.

Possibly dependant on alignment, race and wizard type but options could be:

Meazel, Nothic and Berbalang for that creepy researcher in the Shadowfell and forbidden arts vibe.

Cadaver Collector instead of a shield guardian.

A mercenary detachment of Giff (the hippo-esque humanoids) resplendant in Napoleonic era style uniforms and fire arms.

If you want an "unseen servant" vibe then the previously mentioned Invisible Stalkers and/or Skulk

For a bit of a Far Realm cult following then Star Spawn of various types.

Stone Cursed for that weeping angel vibe.

Boggle for some strangely cute fey servants.

Chromatic Guard Drakes in place of hunting hounds.

Corpse Flower and Assassin Vines for a little garden area.

I would go with some constructs. Gives the impression that this is someone who's powerful enough to simply make their own servants if needed. 

Archmagi are, well as the tin describes, Grand or High Mages, whether of an organisation or otherwise; they could use lackeys, or they could use some form of enchanted/self-made constructs and golems, they could use Necromancy, but that sort of falls into the lane of "I'm a Necromancer, not a Mage", but they are Wizards, so there'd be no harm in an Archmage who slightly implements Necromancy in some capacity.

For "less obvious", you could generally implement anything as long as you do it sensically.

Animated objects or golems, maybe?

smart!

I am a wizard and I have a homunculus

ooh, yeah

Honestly, you can expect just about any type of critter to be employed by an archmage at some point or another, unless it's something that wouldn't be willing to work with them, like an aboleth or a mind flayer.

Really, whatever is best suited to the job.

The moat might actually be a midden full of otyughs, the whole thing cleverly concealed under a programmed illusion.

The only hallway to an entire wing of the castle might secretly be guarded by a gelatinous cube.

They might have a homunculus living in their main Library/Laboratory to act as a Telepathic link to their Spellbook library safely locked in their tower.

They might have steam mephits locked in jars to serve as heat sources for bubbling experiments.

It could be anything.

Arcanoloth. 

Anything that can do menial and probably precise tasks (sufficient Str and Dex so as not to destroy stuff in the lab) and doesn't talk back.

Thanks so much all, definitely some great inspiration!