Here is how to tame Sarco:

1. Craft a bow and tranq arrows

2. Craft Narcotics

3. Find Sarco

4. Sarcos are big, so find a spot where are a lots of rocks, woods...

5. Shoot tranq arrows at it

6. When it falls feed it raw meat or raw fish meat... don't forget NARCOTICS

7. BOOM now you have a Sarco