A Spider is a minor enemy that first appears in the Gloomy Manor of Luigi's Mansion 2. They usually travel in packs of 2 to 5, and are not that much of a threat to Luigi, but still cause him to lose 5 HP if he steps on one by mistake. To defeat a Spider, you need to use your Poltergust 5000 to suck it up, or you can use your Strobulb on the Spider and there is a chance it could drop a coin, but it won't do that in A-5 "Sticky Situation" or A-Boss "Confront the Source", because they can rewpawn in those missions.
A Gold Spider is much more rare than a normal Spider, and is usually found after checking furniture. Gold Spiders travel in packs of 2 to 3, and they only appears on the screen for a few seconds before vanishing, so you have to react fast. If you manage to use your Strobulb on a Gold Spider, it will drop a Gold Bar.