Dominaria Spoiler Digest - Who's Who and What's What from the Release Notes
Tetravus enters the battlefield with three +1/+1 counters on it.
At the beginning of your upkeep, you may remove any number of +1/+1 counters from Tetravus. If you do, create that many 1/1 colorless Tetravite artifact creature tokens. They each have flying and "This creature can't be enchanted."
At the beginning of your upkeep, you may exile any number of tokens created with Tetravus. If you do, put that many +1/+1 counters on Tetravus.
10/4/2004 If the Tetravus is destroyed when the Tetravites are off the card, they are not destroyed, they are just orphaned.
10/4/2004 Any +1/+1 counter which is on this card can be turned into a Tetravite token. It does not care where the +1/+1 counter came from.
10/4/2004 Only Tetravites from this specific Tetravus may be used for the ability. Ones from a different Tetravus can’t.
8/1/2008 This card now has two upkeep-triggered abilities. Its controller chooses the order they are put on the stack, and thus the order in which they resolve each upkeep.