Here's a start, with abilities borrowed from the stirge. I figure we can convert it as normal, then adjust it to be an aquatic creature.
Land Lamprey
Small Vermin
Hit Dice: 1d8+2 (6 hp)
Initiative: +1
Speed: 30 ft. (6 squares)
Armor Class: 13 (+1 size, +1 Dex, +1 natural), touch 12, flat-footed 12
Base Attack/Grapple: +0/+6
Attack: Touch +1 melee (1d3 plus attach)
Full Attack: Touch +1 melee (1d3 plus attach)
Space/Reach: 2 1/2 ft./0 ft.
Special Attacks: Attach, blood drain
Special Qualities: Darkvision 60 ft., scent, vermin traits
Saves: Fort +4, Ref +1, Will +0
Abilities: Str 7, Dex 12, Con 15, Int -, Wis 10, Cha 2
Skills: -
Feats: Weapon Finesse (B)
Environment: Temperate and warm marshes and underground
Organization: Solitary or swarm (2–12)
Challenge Rating: 1/2
Treasure: None
Alignment: Always neutral
Advancement: 2-3 HD (Medium)
Level Adjustment: --
The creature looks like a blackish green worm with a suckered mouth. Concentric rings of sharp teeth line this maw. It slithers along like a snake.
The land lamprey is a mutated version of the sea lamprey. It breathes air and moves in snake-like fashion. They prefer dark and damp environments. Coloration ranges from light green to blackish green.
A land lamprey is only about 3-feet long but fairly thick and heavy, weighing 15 pounds.
Land lampreys do not speak.
Combat
Like their aquatic cousins, land lampreys feed by biting their victims, fastening themselves to the victims with their sphincter-like mouths, and draining blood.
Attach (Ex): If a land lamprey hits with a touch attack, it uses its sphincter-like mouth to latch onto the opponent’s body. An attached land lamprey is effectively grappling its prey. The land lamprey loses its Dexterity bonus to AC and has an AC of 12, but holds on with great tenacity. Land lampreys have a +12 racial bonus on grapple checks (already figured into the Base Attack/Grapple entry above).
Each land lamprey attached to an individual counts as 12 pounds of gear for purposes of carrying capacity and encumbrance.
An attached land lamprey can be struck with a weapon or grappled itself. To remove an attached land lamprey through grappling, the opponent must achieve a pin against the land lamprey. If an attached land lamprey takes even one point of fire damage, it will immediately release its victim.
Blood Drain (Ex): A land lamprey drains blood, dealing 1d4 points of Constitution damage in any round when it begins its turn attached to a victim. Once it has dealt 3 points of Constitution damage, it detaches and slithers off to digest the meal. If its victim dies before the land lamprey's appetite has been sated, the land lamprey detaches and seeks a new target.
SEA LAMPREYS
These cousins of the land lamprey have the aquatic subtype. They dwell in any body of water in any climate. They have a base land speed of 5 feet and a swim speed of 30 feet and are found only in aquatic environments.