English has a long history of borrowing words from other languages--many of which don't follow standard English rules for pluralization. As a result, there are many English words which are normally plural, but don't look plural because they don't end in the letter S. A partial list:
Debris is the only English noun with no plural. There are many English words for which the singular and the plural are the same, but in English, you can not speak of a single piece of debris with a singular form of the word. That is, you would say "Space debris is becoming a menace to satellites," but not "Space debris are becoming a menace."
We often speak of being overwhelmed, or talk of how something might overwhelm a computer, but we rarely use whelm as a verb. Yet in the strictest sense, "to whelm" means "to overcome" (particularly with an emotion); it is therefore perfectly proper to say something like "I was whelmed with grief," which means the same thing as saying "I was overwhelmed with grief."
Both fish and fishes are accepted plurals of the word "fish," but the rules differ as to which one is appropriate in different contexts. Generally speaking, if you are talking about any number of dead fish, or any number of live fish of the same species, you use "fish," as in The boat came back to port with a hold full of fish. However, if you are talking about live fish of different species, you can use "fishes," as in Salt-water fishes include several species of sharks, eels, and salmon.
An alumnus is a male graduate of a school; a female graduate is an alumna. When you're talking about a group of graduates, you use alumni if all the graduates are male or if there's a mix of male and female graduates; if all the graduates are female, the proper form is alumnae.
The plural of mouse is mice only if you're talking about rodents. If you're talking about a computer mouse, the accepted plural is mouses.
The Middle English root of "ruthless" is "ruthe," meaning "compassion." It has survived in the Modern English ruthless ("without compassion"), but its opposite, ruthful ("compassionate") is considered archaic and is rarely used.
There are a handful of other modern English words which are typically used only in their negative form as well. For example, the word scathed means "harmed" or "scorched;" we often use unscathed (as in "I made it through Doom 3 unscathed") but rarely use "scathed." Likewise, the word licit means "conforming to law" or "not forbidden;" we often use it in the negative form illicit (as in "an illicit affair" or "an illicit sale") but rarely describe things as "licit."