Proxy lists aren't published in one universal format, and misreading the format is a common reason a proxy 'doesn't work' when it's actually a formatting mismatch.
The basic IP:Port format
The simplest and most common format is just the IP address followed by a colon and the port, like 123.45.67.89:8080. This is enough for unauthenticated proxies with no username or password required.
Including protocol and credentials
A fuller format specifies the protocol first (http://, https://, or socks5://) and can include authentication as protocol://username:password@ip:port. Leaving out the protocol prefix when your tool expects one is a frequent source of connection errors.
Plain text vs structured exports
Some tools export lists as plain .txt files with one proxy per line, while others offer JSON or CSV with separate fields for IP, port, country, and protocol. Structured formats are easier to parse programmatically if you're feeding the list into a script.