So you want to tame a mosa??? You came to the right place! What you must have

Mastercraft plesiosaur saddle

Plesiosaur

Toxicants arrow

Crossbows

Pro tip compound bow

Plesiosaur saddle free for doing no air pursuit!! Freee

next go around the map a couple times do a tidy save if you can't find one before you explore the plesiosaur cannot be knocked out from jelly fish and dumb eels

Last time I found level 6 moss to bad my toxicants arrows were out :(

... time to do it again