The Wikipedia article on Žaltys says (in its entirety):
A žaltys is a household spirit in the Lithuanian mythology. As sacred animal of the sun goddess Saulė, it is a guardian of the home and a symbol of fertility. People used to keep it as a pet by the stove or other special area of the house, believing that it would bring good harvest and wealth. Killing žaltys was said to bring great misfortunes upon the household. If žaltys was found in the field, people gave it milk attempting to befriend the creature and make it a sacred household pet.
The Britianica article says:
žaltys, in ancient Baltic traditions, a harmless green snake highly respected as a symbol of fertility and wealth. To ensure the prosperity of family and field, a žaltys was kept in a special corner of the house, and the entire household gathered at specified times to recite prayers to it.
On special occasions the snake was asked to the table to share the family meal from their plates; should he refuse, misfortune was imminent. To encounter a snake accidentally was also considered auspicious and portended a marriage or a birth. Paralysis or great misfortune awaited anyone who dared kill a žaltys, the “sentinel of the gods” and a favourite of Saule, the goddess of the sun.
This suggests that it was an actual snake that people would find and keep as a pet and feed milk to. Is there any record of pet snakes being kept in homes for this purpose, or was it likely to have been a snake effigy, carving, etc?
Snakes generally don't drink milk and I assume most snakes wouldn't sit quietly in a corner or share a meal at a table.
If it was an actual snake, was it a particular type/color of snake, or would any grass snake do?