I'm curious what build is EchoPet 2.5.1 as it works with the latest spigot build, the owner of a huge network has assisted me and helped me throughout my problem, so I'm just really curious what build is EchoPet 2.5.1?

NVM! Found spigot build #1657 (latest one available on the internet) and it works perfectly fine! You can close the ticket.

The build you should be looking for is 1649, as per the Spigot forums. There are no later builds available as far as I am aware, only patches. As long as you've verified the MD5 of the build to be official you should be fine :)