There are four main suspects. Remember that witnesses can be suspects as well.

Sinto

Sinto is a fox. He had promised to never harm any living creature in Mossflower and said he was reformed. His house in the forest was normally secretly guarded, but that day the guard was sleeping all day and did not know if Sinto had left. Later it was found that there was a sleeping drug in his drink. Sinto is a suspect because he could have killed Jessie as he didn't like any woodland animal, especially moles, and could have made the poison.

Faykarr

Faykarr is a magpie. He lives deep in the forest and is very dangerous. This bird is wild and uncontrollable. If he decided to hunt out of his territory that night, he could have killed Jessie. This, though, does not explain her poisoning and why he did not eat her.

Ternessa

Ternessa is Sinto's sister. She was found later on the 8th of August near the river and said she was visiting her brother. She could have either found and poisoned Jessie or Sinto could have asked her to kill her. Ternessa is a well-known for making potions and could have made a poison without trouble.

Dayla

Dayla is an otter. She has no serious reason to kill Jessie except for the fact that they had a very big argument a couple of days before. Dayla is quoted as saying "I wish you were dead, mole!". Dayla would probably have needed help with the murder.