If you're smaller than the anaconda, it considers you food. If you're larger than the anaconda, it considers you a lot of food.

Ninth Edition

Anaconda

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)

Swampwalk (This creature is unblockable as long as defending player controls a Swamp.)

You'll never find an anaconda in a swamp. It will always find you first.

Seventh Edition

Swampwalk (This creature is unblockable as long as defending player controls a swamp.)

Urza's Saga

Swampwalk (If defending player controls a swamp, this creature is unblockable.)

Something soft bumped against the rowboat, then was gone.

Portal

Swampwalk (If defending player has any swamps in play, Anaconda can't be intercepted.)

Swampwalk (If defending player has any swamps in play, Anaconda can't be intercepted. Swamps are in play regardless of whether they're tapped or untapped.)