That depends on the model. If objects of that class represent exactly one thing, use singular, if they represent many things, use plural (but you usually do not need such a class, use an array/a collection). Both cannot happen or you should redesign (1).
I'll use your Thread example:
If every object models one thread, name the class "Thread". If it models several threads at once, call it "Threads" (or "ThreadCollection" or the like).
(1) If you need both, a representation of a single thread and a representation of many threads at once, use two distinct classes (Thread and Threads) or create an array or a collection for the latter. Then you'll have it clean:
$thread->show();
$threads->list();