Worth noting that FreeDNS domains can be a mixed bag. Anybody can add or remove the base domains at any time because they’re lended freely. The owner can also arbitrarily decide to delete your subdomain, or reserve it for themselves, or even hijack its use, its TLS certs etc.

The top 7 domains listed there (including mooo.com) as owned by “josh” are somewhat better than the others because Josh is the owner of FreeDNS. So those domains will be around for as long as FreeDNS will, and you know Josh is not gonna hijack your subdomain. But be wary of using any other domain there (or putting your own domain up for use).