If you're in the mood for a ghostly cat, you can try breeding a Ghazt. It's a plasma-wielding Ethereal Monster with feline attributes, and it can produce a zippy electronic sound akin to a keyboard synthesizer.
To get a Ghazt, you'll need to breed it with another Ethereal Monster on Ethereal Island. It's a pretty tricky process, though, so you'll want to stay patient and persistent until you get your Ghazt!
First, you'll need a monster that can combine all three elements of a Ghazt. The best combos are Entbrat and T-Rox, but you can also use any other 3 element Monster that works with Ghazt (T-Rox, Clamble, Pummel, Bowgart).
It can take a few hours before the egg hatches, depending on which parent Monsters you used, so be sure to give it plenty of time. It may even take a few days, but it's definitely worth the effort!
After you've bred the Ghazt, it will be ready to be placed on Plant Island. It can then earn shards and Coins, as well as increase its happiness % and Monster Level.
You'll need to feed it regularly, as it only has a limit on the amount of income it can hold at any given time. This depends on its Happiness % and Monster Level, as well as how much it likes the monster you're feeding it.
You can then sell the Ghazt or place it at Plant Island, Gold Island, or Shugabush Island. It's a great mid-tier Monster for producing coins before you work your way up to the higher-tier Ethereal Monsters.