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Some monsters are listed as being immune to psionics. The most well-known is probably the Tarrasque.

A lesser-known example is the Juju zombie. Does anyone know the justification or rational behind this immunity? For the Tarrasque, I expect it's simply because it's a huge, legendary monster.

But Juju zombies are just undead. Is anyone aware of some reason they would be immune to psionics? I could possibly see them being immune to telepathic psionics, since they have no mind (although this is not strictly speaking true, since they are Int 5-7). But what about their creation would render them immune to psionics as a whole? Their creation process is just Energy Drain from a wizard until dead. Nothing about Energy Drain interacts with psionics in any way I can tell. No other undead (that I'm aware of) are immune to psionics. So where did this come from?

(Juju zombies are also listed as being immune to illusions. I question this as well, but that's another discussion.)

Edit: I just checked the 1e Monster Manual 2 and the Zombie, Juju entry makes no mention of psionics.

Juju zombies are immune to psionics for the same reason they are immune to magic missiles and resistant to fire.

To screw with the players.

They exist so the players can laugh and say, "Hey, they're just zombies," and then get their butts kicked for underestimating them.

Looks like they weren't immune to psionics when they appeared in Monstrous Compendium #1. They only picked up the immunity in the Monstrous Manual.

Re-reading the MM entry a bit more closely:

Monstrous Manual p373 wrote:In addition to normal zombie spell immunities, juju zombies are immune to
mind affecting spells and psionics, illusions, and to electricity and magic missiles.

Psionics is linked with spells to 'mind-affecting'. So they're only immune to mind-affecting psionics, not psionics in general.

Although I still question that 'mind-affecting' part. I think skeletons and zombies pick that up due to not having minds. But ju-ju zombies have Low Intelligence (5-7) and clearly retain some of their previous knowledge. So they clearly still have some kind of mind in there, which should be contactable by a telepath.

I did read it as mind affecting applying to both spells and psionics. And I agree with Jaded's assessment. The intent is to surprise players, but not everything has a specific explanation in their creation (i.e. the fire and MM immunity)

It doesn't say they are immune because they have no minds, only that they are. It is admittedly a rare exception