It is a combination of a rat and a mosquito bred and mutated. There are many types of Mosquito Rats, but the basic kind is probably how you vision it to be.
They actually scare younger children with the stories that they suck Diarrhea out of your asshole. These stories are in fact, TRUE.
Younger ones tend to miss the asshole, and go for your eyes or ears, or any other hole in your body. They use a long tentacle-like organ to suck with.
Younger Mosquito Rats are very small compared to the varied sizes of much larger adult ones. Babies are the size of a normal Mosquito, adults can range from the size of a dime, to the size of a school bus.
They usually reside in basement walls, and in rotting food at the grocery store.
It is also possible to take them home in your hair if they are infested at another location. Mosquito Rats will also crawl through other parts of your houses' walls, making scratching noises to scare children so they can make a diarrhea strike.

They CAN fly.

When you eat dirt for the first time and leave your family to build a shack in the woods