You may sacrifice only creatures that are already on the battlefield. If a creature with devour and another creature are entering under your control at the same time, the creature with devour can't devour that other creature. The creature with devour also can't devour itself.
If multiple creatures with devour are entering under your control at the same time, you may use each one's devour ability. A creature you already control can be devoured by only one of them, however. (In other words, you can't sacrifice the same creature to satisfy multiple devour abilities.) All creatures devoured this way are sacrificed at the same time.
You may choose not to sacrifice any creatures for the Devour ability.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Raymond Swanland
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The number of Saproling tokens created by the triggered ability is based on the number of +1/+1 counters on Mycoloth, not on the number of creatures Mycoloth devoured. It doesn't matter where the +1/+1 counters came from.
