Nesset

From Memory Alpha, the free Star Trek reference.

The nesset is a small animal native to the Nechani homeworld.

According to the beliefs of the Nechani, the nesset is able to travel from this world into the spirit realm where the nesset serve as gatekeepers. In order to travel into this spirit realm, one has to be bitten by a nesset.

Captain Janeway put her hand into a basket with a nesset and let herself be bitten, in order to save Kes' life. The nesset bite caused a poisoning that led to a burning sensation in Janeway's chest, but she was in no immediate danger. (VOY: "Sacred Ground")