I had the same thought as Luciano: {petaQ} may have originally been some sort of disgusting creature or thing, which is why it’s pluralized with {-mey} in the same way that {DeSqIvDu’} “elbows” is also used for V-shaped pot handles (KGT p.97).  But as SuStel says, we have no way of knowing.

For comparison, more than one animal - even when used to refer to people - is {Ha’DIbaHmey} in the paq’batlh:

ghoStaHvIS tam 'ej So'
   molor QaS HoS
   ram Ha'DIbaHmey rur
They sneak and they creep,
   The men of mighty Molor,
   Like beasts in the dark.  (PB)

DaqtaghlIj yIlel qeylIS
   ngemvamDaq Ha'DIbaHmey tIHoH
   wa' targh cha' tlhInganpu'qoq je
Kahless, pull your d'k tahg,
   Kill the beasts in this forest,
   One a targ, and two who call themselves Klingon. (PB)

Note that {Ha’DIbaHmey} refers to two {tlhInganpu’qoq} “so-called Klingons” in the second example. 

--
Voragh

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From: SuStel

On 9/27/2019 11:03 AM, Luciano Montanaro wrote:

As far as I know, petaQ is an insult, but the specific meaning has
been withheld, probably to make it safe to use in a program for
children.
As such, we cannot desume that using -mey is an additional insult; as
far as we know, a petaQ may be a disgusting animal, or thing, so -mey
would be the grammatical correct way to pluralize it.
Am I missing something here?

I agree. We just don't have any data on which to speculate. All we know is that in paq'batlh Molor calls a group of assembled Klingons petaQmey.