import re


def is_valid_cg_id(cg_id):
    """
    Validates a consistency group ID, also known as the consistency group
    "cg_name".  A valid consistency group name should look like: cg-00000001
    - It should always start with "cg-" and end with 8 hexadecimal characters
    in lower case.

    :type cg_id: str
    :param cg_id: The consistency group name to be validated.

    :rtype: bool
    :return: True or False depending on whether cg_id passes validation.
    """
    if cg_id is None:
        return False

    match = re.match(r'^cg-[0-9a-f]{8}$', cg_id)

    if not match:
        return False

    return True