I seem to remember the company behind either Monster Cables, or Monster energy drinks, going after anyone that used the word “monster,” even in casual context.

The most ludicrous case of trademark issues I've seen to date has been Apple suing Prepear because they had a pear as their logo but Monster suing someone because of the term monster is not far from falling equally as far from the tree of wisdom and common sense.

Every large company does this to some extent, because thre's a concept in US trademark law that if you don't aggressively and pre-emptively defend your mark then 'constructive abandonment' becomes a valid defense against infringement. That means people can rip off your trade mark and then say in court 'well, I thought he company had given up ownership of the trademark because they didn't sue 'monster plush toys', so I decided to call my energy drink 'Monster Brew.'' This is also why US companies slap a 'TM' next to every instance of their trademark, even though it becomes a visual distraction in graphic design terms. Lawyers will argue that if you don't aggressively police the bounds of your intellectual property at all times then it evaporates.

It was Monster Cables who, blessedly, are nearly irrelevant nowadays. Apparently they tried to pivot into online gambling a few years ago.

or Facebook going after anyone with ...book in the name.

See also: Edge Games https://en.wikipedia.org/wiki/Edge_Games#Trademark_disputes