The yutyrannus, not the rex, is your best friend for leech hunting. The leech is vulnerable to the fear roar, and the debuff reveals their location. This makes hunting them a simple errand. You'll collect so much leech blood you could start a swamp fever clinic.
I'm used to them just like getting on my body and just going into fire, but anyways I went out to look for something to tame in the swamp and I actually saw one like in the water and not on anything ( I was on my imprinted Baryonyx ) and I just turned into a cat and started hissing at. it like actually hissing and I just decided "that's it I'm going back" but the way it was moving in the water it was just terrifying and disgusting