There is a semi-secret Ringtail Slime in the Starlight Strand. It is easily accessible with no upgrades as soon as you warp in.
Unlocking the Starlight Strand
To unlock the Starlight Strand, you will need to feed the Cotton Gordo at the Rainbow Fields. This will require 30 vegetables of any type. It favors Water Lettuce and each will be worth two regular vegetables when fed. Carrots grow wild everywhere in the Rainbow Fields and there is a small patch of Water Lettuce just behind the Gordo.
Finding the Ringtail Alter
Upon entering the Starlight Strand, travel the “blue side” until you reach an area with a temple with a closed door.
Take the opening along the right and as you descend the slope, turn to your left to see the “Rightail Alter.”
Enjoying your Ringtail Slime
It is not guaranteed, but more often than not, if you are vising during the day, a single Ringtail Slime, in statue form, will be waiting for you. You can use your vac to carry it to a safe location, or wait until night for it to become a slime again, then store it in your vac tank.
Here is the location on the minimap.
Occasionally, if you are in the area for awhile, more than one will spawn.
If one does not spawn, hang around the surrounding area for awhile or come back later.