A position is described as a beast when it is particularly challenging to
- somebody (or some algorithm) trying to analyse it, e.g. to find the best line of play or to evaluate it as a combinatorial game or
- the (makers of) rules (in which case it is a rule(s) beast), in the sense that one or more rule sets imply results that players find dissatisfying or counter-intuitive.
But sometimes beast is just a slangy way of referring to any (moderately interesting) group or position.
A collection of beasts is often referred to as a bestiary.
Most rules beasts have been specially constructed, but some have occurred in real games; they are in any case rare, and so barely affect most players. In either case, they are sometimes used in rules disputes to demonstrate a point.
A nasty, mean spirited opponent can be described as a beast.
- BQM 222 A difficult pro game situation (Lin Sakata, 22nd Honinbo game 4)
- More Infinitesimals Some infinitesimals in chilled go with multiple live options.
- Hanezeki A seki where both sides decline to capture one or more stones in atari, one of them perhaps a hane on the first line
- A pathological seki An apparent seki where two black groups share four liberties each with both of two white groups, which arose as a test-case for an algorithm
Rules beasts (or suggested to be such)
- Repetition rules
- Cycle Positions that return (at various lengths) to their starting point
- Rules Beast 1 superko surprisingly forces a connection in a seki to avoid a capture (John Tromp)
- Superko puzzle 2 Another superko puzzle
- Sending two, returning one A cycle that transfers points in territory scoring
- Pinwheel Ko In which kos rotate around the board, indefinitely in the absence of a superko rule
- Unremovable ko A ko which one player can start whenever they like, as their opponent cannot remove it
- Unremovable ko for both sides A ko which neither player can remove
- Triple ko Infamously considered a draw in some rule sets without superko
- Leather Scroll Problem 64 / Solution Bill wonders if this old triple ko problem was intended as a rules beast
- Eternal Life (chosei) A particular cycle
- Spight Rules/Example A ko both sides want to win but neither wants to fill
- Capturing rules
- 1-eye flaw An uncapturable group with one eye
- Moonshine Life Referenced in a historical rules dispute: a group with a false eye in ko that cannot be captured because the owner has unlimited ko threats
- Sections Seki with one… and …two false eye(s) of page Seki Cases of seki scored differently in different rule sets
- Seki without shared liberties Test cases by Matti Siivola for the no territory in seki rule
- Suicidal Tendencies Analysis of an item from Harry Fearnleys bestiary (link below) exploring consequences of allowing suicide
- Three points without capturing (Torazu sanmoku) Japanese rules specified the result without it having to be played out
- Five points without capturing (Torazu gomoku) Japanese rules specify the result without it having to be played out
- https://www.cs.cmu.edu/~wjh/go/rules/siivola.html Four rules beasts by Matti Siivola
- http://www.helsinki.fi/~msiivola/tilanne5.html A fifth rules beast by Matti Siivola (also in article Matti Siivola as of 2019-01-30)
- http://www.harryfearnley.com/go/bestiary/ Harry Fearnleys collection of beasts
- http://denisfeldmann.fr/bestiary.htm - Denis Feldmann's bestiary. To quote its introduction, it is a collection of "Strange situations, famous tesujis and other wonders". In English, but also available in French.