Krokodil is low quality, contaminated desomorphine. Very easy to make. Not sure how popular it is right now, surely less than 15 years ago. Eats your skin away because it's so contaminated, but i'm 99% sure for that to happen you need to miss a vein first.
Tranqfent is fentanyl cut with xylazine or some other sedative. I don't know much else about it.
Both objectively suck and were made for addicts that wanted more/cheaper product, by addicts(?). But you can only make one of these in your garage （⌒∇⌒ゞ）