I believe it's called the intrusive 'r'. The upside of it is that it makes the following joke work:

What do you call a one-eyed dinosaur? A dyathinkhesawus. Without the intrusive 'r' this joke would not work.

I moaned about this on https://english.stackexchange.com/questions/317198/the-r-sou.... Some good answers to the linked dupe.

Did not know about that discussion. As a "rhotic" person it makes me wince but I accept different accents. In any chat that leans towards getting heavy on this I refer people to The (great) Proclaimers song: https://www.youtube.com/watch?v=066oSmDRKPA