EchoPet wont load with latest Spigot #1646 build with 1.8 API and all plugins updated. Tried with both latest and oldest version of EchoPet, still doesn't seem to work.

There is an announcement on the BukkitDev page which explains exactly why this is happening:

If EchoPet is not loading on your Spigot 1.8 build and your server is outputting "Skipping loading of EchoPet as it does not work with Spigot 1.8 builds!", please try running a later Spigot patch. I can't do anything about older Spigot releases blacklisting the plugin (preventing it from loading).

This is not something I can fix, as certain builds of Spigot prevent EchoPet from loading.

[20:46:10] [Server thread/INFO]: Starting minecraft server version 1.7.10
[20:46:10] [Server thread/INFO]: Loading properties
[20:46:10] [Server thread/INFO]: Default game type: CREATIVE
[20:46:10] [Server thread/INFO]: This server is running CraftBukkit version git-Spigot-1646 (MC: 1.7.10) (Implementing API version 1.7.10-R0.1-SNAPSHOT)
[20:46:10] [Server thread/INFO]: Debug logging is disabled
[20:46:10] [Server thread/INFO]: Server Ping Player Sample Count: 12
[20:46:10] [Server thread/INFO]: Using 4 threads for Netty based IO
[20:46:10] [Server thread/INFO]: Generating keypair
[20:46:10] [Server thread/INFO]: Starting Minecraft server on 188.165.255.166:25565
[20:46:11] [Server thread/WARN]: Skipping loading of EchoPet as it does not work with Spigot 1.8 builds!

I do understand what you've said but this is the latest spigot build available on the internet lol.