Hapatra, Vizier of Poisons

Whenever Hapatra, Vizier of Poisons deals combat damage to a player, you may put a -1/-1 counter on target creature.
Whenever you put one or more -1/-1 counters on a creature, create a 1/1 green Snake creature token with deathtouch.

Pit Scorpion

Whenever Pit Scorpion deals damage to a player, that player gets a poison counter. (A player with ten or more poison counters loses the game.)

Serpent Generator

{4}, {T}: Create a 1/1 colorless Snake artifact creature token. It has "Whenever this creature deals damage to a player, that player gets a poison counter." (A player with ten or more poison counters loses the game.)

Glistener Elf

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

Crypt Cobra

Whenever Crypt Cobra attacks and isn't blocked, defending player gets a poison counter. (A player with ten or more poison counters loses the game.)

Seshiro the Anointed

Other Snake creatures you control get +2/+2.
Whenever a Snake you control deals combat damage to a player, you may draw a card.

Obelisk Spider

Reach
Whenever Obelisk Spider deals combat damage to a creature, put a -1/-1 counter on that creature.
Whenever you put one or more -1/-1 counters on a creature, each opponent loses 1 life and you gain 1 life.

Marsh Viper

Whenever Marsh Viper deals damage to a player, that player gets two poison counters. (A player with ten or more poison counters loses the game.)

Nest of Scarabs

Whenever you put one or more -1/-1 counters on a creature, create that many 1/1 black Insect creature tokens.

Swamp Mosquito

Flying
Whenever Swamp Mosquito attacks and isn't blocked, defending player gets a poison counter. (A player with ten or more poison counters loses the game.)

Sabertooth Cobra

Whenever Sabertooth Cobra deals damage to a player, that player gets a poison counter. The player gets another poison counter at the beginning of their next upkeep unless they pay {2} before that step. (A player with ten or more poison counters loses the game.)