"Play horse poker" likely refers to H.O.R.S.E. Poker, a popular mixed-game poker format often played in high-stakes cash games and tournaments.

What is H.O.R.S.E. Poker?
H.O.R.S.E. is an acronym for five different poker variants played in rotation:

H – Hold'em (Limit Texas Hold'em)

O – Omaha Hi-Lo (also called Omaha 8 or Better)

R – Razz (7-Card Stud Low)

S – Seven Card Stud

E – Seven Card Stud Eight or Better (Stud Hi-Lo)

Each game is played for a set number of hands or time period before switching to the next game in the sequence.

How Do You Play H.O.R.S.E.?
Set the structure: Decide how many hands (e.g., 8 hands per variant) or time (e.g., every 20 minutes) each round will last.

Play Limit-style games: All games in H.O.R.S.E. are typically limit games, meaning the betting is fixed rather than no-limit or pot-limit.

Rotate the games in order:
Hold'em: Standard Limit Texas Hold'em.

Omaha Hi-Lo: Four hole cards, must use two, and the pot can be split if there's a qualifying low.

Razz: Seven Card Stud lowball — the lowest hand wins (A-2-3-4-5 is the best).

Stud: Standard Seven Card Stud high — best five-card poker hand wins.

Stud Eight or Better: Split pot between the best high and qualifying low hand (eight or better).