Is monomania a real thing? if so, what is it?

Its an archaic term: coined by a psychiatrist in the early 19th century. Here's a good lay reference: https://en.wikipedia.org/wiki/Monomania Its not used anymore, and if it was it would be unipolar mania, meaning altered mood in one direction--UP, or manic. It was a more complicated term in 1810 and does not comport with the underlying dynamics and cause(s) of bipolar disorder, which we better understand now.

Monomania: is fascinating, and refers to delusional states that are confined to a single subject or an emotional state. There is an example, shared delusion where a couple (usually married) will share a fixed delusion, and who are otherwise not psychotic outside of the delusion. Although monomania probably exists where sufferers are not otherwise ill, it would be exceptionally rare. Great question!