Board Thread:Questions and Answers/@comment-12794005-20131013131100/@comment-24026943-20140805064454

Cosmo was a Sonic X exclusive character, Sonic X is over (sadly), but many people predicted what Season 4 would be if it was made, a common prediction is that the death of Cosmo. Puts Tails in a state of depression and anger which results in Dark Tails.

Also, Cosmo doesn't exist outside Sonic X, so unless it gets a revival, don't expect any thing to do with Cosmo.