I thought I had no hope of ever finding the rules on the internet two decades later, but a few clever googles later, I found them: it was called Food Chain.
The main thing I thought was interesting about it is the various basic attributes (which turned out to be Nose, Eyes, Ears, Mouth, Paws, Tail, Legs, Brain, no SPEED or CLAWS in sight after all) determine, as derived facts, both your critter's size and the size-range of things they try to predate on. I liked that tension between the things you get to choose, and the maybe-unintended consequences that follow from them. Especially because the maybe-unintended consequences aren't *explicitly* bad, but depend very sensitively on which other critters are in the population at that moment. It's not just an RPG with some Advantage Points and Disadvantage Points you have to distribute, but something that feels subtler --- even though it's still a very, very simple game, mechanically.
If ever there were a few sentences so perfectly native to those lovely, naive, early days of the internet, they would be these:
Whenever you start a new species with this order, any old species currently under your control in the same game are eliminated. You could cheat here by sending in your START order using a different email account. Please don't.