I'm surprised nopony else thought of this or made a topic similar to this, this is a fairly common game in other forums.
So the general rule is to reply with a word that relates to the last post in some way. If you can't think of anything, you just type in something else to keep the game going.
Example:
Pony 1: Winter
Pony 2: Snow
Pony 3: Mt Everest
I'll start: Summer
Heat
Sun
Celestia
Pancake
Cupcake
Pastry
Pie :3
Circles
Ring
Lord of the Rings
Books! ^-^
Twilight Sparkle
Rarity
Dresses! ;)
Fashion
The ArtsTM
Daring Do
Rainbow Dash :3
Clouds
Rain
Mud
Dirt
I knew that was coming
Soil
Pants
You can guess why.
Shoes
Boots
Socks
Legends of Equestria
Friendship
Elements of Harmony
equanimity
Equine
Ponies
Horses
Animal :3
Dogs
Cats
Lion
Mountain
Rocky
Boxing
Canada
America
Donald Trump
The Apprentice
The Ranger's Apprentice
Let's bring this back :3
Ranger
Fighter
Boxing
Sport
Football
Grass
Goat
Llama
Wool
Sweater
Warm
Cool
Sunglasses
Sun
Tan
Pale
Dracula
Fangs
Talon :3
Predator
Ecosystem
Biocenosis
Life
Health
Weather
Water
Humidity
Meteorology
SCIENCE!
Woona.
(I'm going to guess that's Luna)
Night
Stars?
Star Wars
Laser Pistol
Light beam
Death Star
Darth Vader
Dads
(Luke, I am your father ovO)
Sandwich
Burger
Nutrition
FOOD
quesadillas
Cheese
Dairy
Cow
Sheep
Sheepdog
puppies ono
Boxers
Boxing Gloves
Leather (what? Boxing gloves are gloves, and gloves are usually made of leather to me)
Archery armguard (because Leather armor is that typical archer armor in games)
Bow (because archery, ok)
Arrow
Road signage
Cars
Cars (the movie) lol
'Mater
Truck