In the Scooby Doo cartoons, the main character Scooby Doo has a speech disorder, what is it called?

Scooby's speech disorder is called rhotacism, which is characterized by excessive use of the letter "R".

Rhotacism may refer to an excessive or idiosyncratic use of the letter r, the inability to pronounce (or difficulty in pronouncing) r, or the conversion of another consonant into r.

In medical contexts, rhotacism is the inability or difficulty in pronouncing the sound r. Thus, many speech pathologists call this problem de-rhotacization, because the sounds lose their rhotic quality rather than becoming rhotic.

The rhotic sounds are usually the last ones a child masters. Some people never learn to produce them; they substitute other sounds, such as the velar approximant, the uvular approximant (often called the French R), and the uvular trill.

In English, the most common occurrence of this type is a pronunciation perceived as closer to (typically, though, actually the labiodental approximant), which is known as r-labialization. This form of rhotacism has often been used in English-language media for comedic effect, since it evokes among English speakers a childlike way of pronouncing the letter R.

Rhotacism is more common among speakers of languages that have a trilled R, such as Swedish (except in the provinces of Skåne, Halland, Blekinge, Öland and southern Småland), Finnish, Italian, Polish, Romanian, Russian and Spanish. Many Quebec rural regional accents along the St. Lawrence corridor are characterized by rhotacism.