I’m not an expert, but how I understand it is that because the plural -lar has been already used with Kaplumbağa, so there’s no need to use ‘double plural’ (as with ‘yer').
you can say, “Kaplumbağalar peynir yer”
you can also say, “Kaplumbağa peynir yerler” (this applies to animals and humans only)
However, if you’re talking about cars or flowers for example, you should use the following pattern;
Arabalar güzel/Onlar güzel
or çiçekler güzel
As long as there is “-ler/-lar” in the sentence, it’s clear that we’re talking about ‘plural’ whether it’s used with the noun or with the verb (BUT don’t use it for BOTH in the same sentence).
Please correct me if I’m wrong :)
And for the people who felt the need to downvote my explanation, my source is this:
I'd be more than happy to hear about different opinions, though.
Nothing, in modern Turkish.
Kaplumbağa - Turtle
"Kap" is a cover, lid, shielding material. So one could argue that "kaplu" derives from "kaplı" → shielded, but as an isolated word "kaplu" doesn't mean anything.
It seems that "bağa" used to mean "frog" in ancient Turkish. So that makes it a "shielded frog". But "bağa" is not a word anymore, and frog is kurbağa, whose etymology, as far as the "kur-" part is concerned, is unknown.