Some sort of homebrew giant serpent, serpent swarms, possibly cockatrices. Or even (stretching things a bit) a chimera or dragon. Not entirely sure what you mean by your last question? You can vary encounters based on what purpose they serve in the dungeon. Maybe the Yuan-ti keep some serpents around in an area to help in snake-worshipping rituals. One monster might be a pet. Others patrol the place. A snake-swarm or group of vipers might be part of a (pit) trap. Another might just have wandered into an abandoned section and remains there, tolerated by the intelligent enemies because it is vaguely serpent-like and thus holy. Encounter settings might include pillared-halls, ornamental jungle-gardens (possibly to provide sustenance for a walking/bleeding food-source), magical sanctums, torture-chambers, barracks, temple halls, ruined areas with cracks or deliberately constructed tunnels that are suitable for a snake-creature but too narrow for larger/rounder creatures, fortifications, bathing pools.
You can make environments that cause tactically relevant effects - pool, rivers, rooms divided by grates that ranged attackers can fire or cast spells through, narrow tunnels, areas with lots of alcoves / pillars / statues for stealthy creatures to hide behind (very effective with medusae), elevated areas, regions with magical effects - for instance maybe an area-wide curse that imposes disadvantage on saves vs poison. Maybe an alchemy lab in one section with amphorae of experimental poisons that enemies can use against PCs to cause diverse effects or ailments. Maybe an indoor moat in one place filled with poisonous or unbreathable gas. Intelligent enemies can construct castle-style fortifications with portculli, murder holes, and/or indoor partial-height walls to take cover behind.