Unusual Creatures to Challenge A Party

[Palanan]

I’m running a game for a third-level party, and for story reasons I’ve been throwing primarily humanoid foes at them. But I’d like to mix it up and send something peculiar, challenging and disturbingly memorable after them.

Right now the party is a little understrength—we have a druid who’s hopeless in melee and relies on his animal companion, a monk who’s flurry-challenged and a wizard who mainly serves as support for the monk.

I’m looking for oddball but deadly creatures that could pose a strong challenge to my hapless party. I’d like to keep this to official Paizo sources, but the more obscure and unusual the better. What could I use here?

[Elvensilver]

How about a Sarcovalt? (https://aonprd.com/MonsterDisplay.aspx?ItemName=Sarcovalt)

Only used them once, but they have many interesting abilities (detach head! Vomit swarm! Lesser animate dead!) and stuck in the players' minds. Even if the wizard makes their knowledge check and learns some, they have so many abilities that they will continue to surprise the party.

With damage reduction 5/good or silver and some resistances, they are rather sturdy, especially against monks or certain animal companions who rely on multiple weaker attacks. Then again, they are only CR4, so not exactly deadly for a level 3 party. Maybe bring two, or add some freshly raised undead?

[Septimus]

I recently use a weird one, the derro magister (https://www.d20pfsrd.com/bestiary/monster-listings/humanoids/derro/derro-magister/), against my most recent group. The combat was memorable even so it was one-sided due my bad luck. The magister has some weird abilities that will surprise your party, but probably not endanger it too much. With debilitating spell-likes, mesmerist stare, poison use and sneak attack, it is full of surprise. Add a simple friend derro (for the flanking) for a CR of 6.

[Palanan]

Both of these suggestions are great, very much appreciated.

Open to other ideas along these lines.

[Beni-Kujaku]

Remorhazes are always a nice monster, with a bit of puzzle in it due to the "destroy weapons" thing. Though it can be a very caster-centric fight with martials mostly relegated to tanking.