The Leech is a Xenian species, introduced to Earth during the Resonance Cascade at the Black Mesa Research Facility. It is yellowish-gray in color, and has minuscule, razor-like teeth. It is very similar to the terrestrial species of leeches found on Earth.
Leeches are not present in the first release in Black Mesa, but the developers are considering adding them back in as harmless prop NPCs.
Although they are not present physically in the mod, diagrams of Leech anatomy appear on a whiteboard in Questionable Ethics. Labeled organs include a "Gas Bladder," "Ganoid Scales," and "Cuticular Teeth."
Leeches live in aquatic environments. Being carnivorous, they mount weak attacks on organisms that happen to come into the water. Individually, they pose little threat, but can be damaging when they attack in numbers. They are very frail, and will die with a single attack, whether melee, explosives and/or firearms are used.
Early in development, the Half-Life 2's leeches were originally meant to attack the player like in the original Half-Life, but adjusted to fit the Black Mesa's gameplay.
They were spawned randomly around the player in water, when a cvar "env_leech_controller" was activated in the map. The leeches behaviour was tweaked in a way they only hurt players that are not swimming, to invoke a sense of tension while not overwhelming a player with low health. Also, they were designed to only lower the player's health to 10%. Both of these options could be tweaked by console variables ("sk_leech_dc_player_static" and "sk_leech_dc_player_max_health" respectively).
Later, play-testing shown this was highly confusing to the testers. As revealed by Nathan Ayres, "[the leeches] were making the underwater sections, especially Apprehension, too difficult. It's already hard enough to explore underwater in near pitch blackness, it was too much to have your health also being eaten away by creatures you could barely see and were too quick to kill. Playtesters didn't understand why they were losing health and once we told them, they routinely wasted all their ammo trying to shoot the critters, in a chapter with not much ammo. "
Because of this, the leeches were scrapped in the first release.