It's true that "tongs" sounds plural, but it's actually a mass noun or collective noun. These nouns refer to a group of things that are considered a single unit, even though they are made up of multiple parts.
Here are some other examples of mass nouns:
* Scissors: Although it has two blades, it's considered a single tool.
* Pants: While it has two legs, it's considered a single garment.
* Pliers: Made of two parts, but functions as a single tool.
Therefore, you wouldn't say "a tong," just like you wouldn't say "a scissor" or "a pant." You'd use the plural form "tongs" to refer to a single pair of tongs.