Money Bag

Moneybags are enemies in Super Mario 64. They are green wallet-like creatures that disguise as coins. When Mario gets close, their true form can be seen, and they start hopping around and damaging Mario if they touch him. They drop several coins when jumped on. They can only be found in Snowman's Land.

Cool, Cool Mountain Manifestation
Several reports have stated Money Bags can appear in Cool, Cool Mountain, usually with the same behavior. This likely happens due to Cool, Cool Mountain being similar to Snowman's Land, as well as the fact the files reference Money Bags to appear in Cool, Cool Mountain, despite never actually being seen there in standard copies.

Hungry Money Bag
Hungry Money Bags are an anomalous variant of Money Bags that reportedly steal coins, with the taken coins either being dropped upon defeat or reappearing in the level. Some reports even state the Hungry Money Bags stole stars that would usually have to be re-collected in the levels they came from, though may be dropped by the Hungry Money Bag instead.

Trivia

 * With good timing, it's possible to make a Money Bag duplicate itself by interrupting it during a few frames of its transition from its enemy form to its fake coin form. Exploiting this glitch can allow the player to obtain infinite coins in Snowman's Land.