Snape grass
Snape grass can be obtained when Fishing manta rays, this requires a fishing level of 85. Snape grass will appear in your inventory while fishing manta rays just like Caskets do when fishing at Catherby. It is only used as an ingredient to make Prayer and Fishing potions by using the Herblore skill.
There are also a couple monsters where you can obtain snape grass from.
Prayer potions are created by using the snape grass on an Unfinished rannar potion. This requires a herblore level of 35 and will grant 440 experience.
Fishing potions are created by using the snape grass on an Unfinished avantoe potion. This requires a herblore level of 56 and will grant 565 experience.