The murtlap is a bit of an oddball in the Harry Potter world, an aquatic rodent that produces a substance that is a common ingredient in some healing potions. We do see one of these creatures briefly in Fantastic Beasts & Where to Find Them but that is about it. With that in mind I gave it a few adaptations and made it a truly aquatic rodent with the ability to breathe underwater, swim at decent speeds, and to see in the dark (it probably isn’t a deep sea dweller, but even at higher depths light can be scarce underwater). Essence of murtlap is a restorative but undiluted it can be harmful, I represented that with the murtlap’s sting, which won’t be difficult for most characters to ignore and isn’t deadly by any stretch of the imagination.
Small beast, neutral
Armor Class 12 (natural armor)
Hit Points 20
Speed 25 ft., swim 25 ft.
Str 10 Dex 12 Con 10 Int 8 Wis 10 Cha 8
Skills Stealth +5
Senses darkvision 60 ft.
Amphibious. The murtlap can breathe air and water.
Bite. Melee Weapon Attack. +1 to hit, reach 5 ft., one target. Hit: target takes 3 (1d6+2) piercing damage.
Claw. Melee Weapon Attack. +1 to hit, reach 5 ft., one target. Hit: Target takes 2 (1d4+1) piercing damage.
Sting. Melee Weapon Attack. +3 to hit, reach 5 ft., one target. Hit: Target takes 1 piercing damage and must make a DC 8 Constitution saving throw, on a failed save the target is poisoned for 12 hours.