Abdul Hakim al-Shishani, a veteran of the Chechen-Russian war in the 1990s and commander of the Ajnad al-Kavkaz group fighting Syrian government forces in Latakia, arrived in Ukraine with a group of Chechen fighters to battle Russian forces.

Chechen fighters leave Syria to battle Russians in Ukraine

Related:

ISIS Fighting Russians in Ukraine

ISIS is a western proxy. It’s a NATO irregular mercenary army. It functions as a sort of ‘stay behind army’ along the same vein of what was created by NATO after WW2. These stay behinds may or may not include special forces. The facts of that being the reality speak for themselves.