Grindylows: 

Grindylows are small, green water demons with the height of 3-5 feet and horns on their head. Their diet consists of fish and algae, but they prefer fish. If needed for studeies, Grindylows can be kept in tanks for a short period of time, but need foliage and fish. They have quite a negative attitude towards humans, however they can peacefully coexsist with merpeople.

Grindylows are able to mate and give birth, usually to twins. These babies are called Grindas or Beba and feed on algae until they are able to transition to fish. You can determine the age of a Grindylow by looking at the amount of rings on its horn. They usually live anywhere between 8-10 years. Grindylows create sparks and moving objects using magic. They often use this for hunting or a warning sign. 

Dark creatures: 

A dark creature is defiened as a creature which feeds on humans. It's important to understand that dark creatures are not associated with dark magic. For Grindylows, their last mailing occurred in 1679, however there have been minor attacks since then. You can use the Revulsion Jinx as a defence, ensure you keep your distance and make it clear you aren't a threat.