A lot of entities have been tweaked in SE. Here's a list of most of the changes.

Some entities now also change along time. You can check said progressions here.

New entities

Fire Springs

Natural Hazards that generate in Metamorphic Caves.

Sculk Plantoids

Plant-like entities, voraciously hungry for Xp.

Vanilla changes

Blazes

Will now spawn on Metamorphic Caves and nether fortresses.

Bogged

Can be obtained from a rotting bogged zombie.

Baby Bogged now exist. They either spawn on generation or from a rotting baby bogged zombie.

Creepers

Will spawn with a configurable random fuse time.

Drowneds

Can spawn wearing copper helmet and chestplates, some of them charged. Can also spawn holding tridents and fishing rods.

Baby Drowneds will hold big tridents unobtainable to the player.

They will drop Humid Rotten Flesh when killed.

Unlike in Vanilla, Drowneds are now weak to the Impaling Enchantment.

Endermen

Can spawn already holding natural blocks from all three dimensions. They will only hold blocks from the dimensions the nearest player has been to.

Frogs

Will also eat silverfish and endermites. They can drop [Frog Legs] (https://github.com/CiscuLog/Survivor-s-Elegy/wiki/Frog-Leg) on death.

Guardians

A blue discharge will appear when a Guardian or Elder Guardian attacks a Player.

Hoglins

A Hoglin killed by a charged creeper will drop their head.

Hoglins will not zombify naturally outside the nether. Alternatively, they can be zombified by being inflicted the Phage effect.

Husks

Can spawn holding a wide variety of items and weapons. They will drop Dry Rotten Flesh when killed.

Iron Golems

Walk nearly as fast as on land when underwater.

Item Frames

Will be invisible when displaying certain items (shields, hoglin heads, zoglin heads, shutters and bee counters).

Illagers

Will hold torches on their offhands if the area they're in is dark enough.

Phantoms

Phantoms will disappear after spawning if the nearest player has a cat or ocelot in a 32 block radius.

Pigs

Will give birth to a litter between 1 to 4 piglets.

Piglins

Piglins will spawn wearing new gear and weapons, such as custom piglin Shields.

Piglins will not zombify naturally outside the nether. Alternatively, they can be zombified by being inflicted the Phage effect.

Piglin Brutes

Will Spawn using headgear and new unique axe designs.

Pillager Patrol Leaders

Will drop a Patrol Leader Compass when killed by a player.

Players

Will drop their heads when killed by a charged Creeper.

Skeletons

Will spawn holding a wide range of weapons, as well as shields. Will only drop arrows if holding a bow.

Piglin Skeletons can be found in the nether wearing piglin gear, and Villager Skeletons can spawn both on the overworld and rarely the nether.

Baby Skeletons now exist. They either spawn on generation or from a rotting baby zombie.

Strays

Can be obtained from a rotting stray zombie.

Baby Strays now exist. They either spawn on generation or from a rotting baby stray zombie.

Villagers

Nitwits no longer exist.

Novice Fishmongers can sell any fish.

Expert Cartographers will sell Dark Cave Explorer Maps leading to Ancient Cities. Master Armorers will sell Chainmail Horse Armor.

Fletchers have been nerfed and will no longer trade emeralds for sticks. This can be toggled in the config menu.

Librarians have been nerfed and no longer will trade mending books. This can be toggled in the config menu.

Wandering Traders

Will always sell Enchanter Guides. They will also announce their presence by playing a goat horn (up to version alpha 0.8.7).

When killed by day, they will drop an invisibility potion. When killed by night, they will drop an empty bottle.

They can spawn with one of the llamas unleashed. On other instances they will spawn not with two llamas, but with a charge animal and a pet instead.

Holding an Ominous Banner near a Wandering Trader will enable the Clandestine Market

Trader Llamas

Will spawn chested and wearing different carpets.

Sometimes one of the llamas following Wandering Traders will be unleashed, forming a caravan with the leashed llama.

Wardens

Wardens will try to bury themselves and despawn when having low health.

Playing goat horns will instantly anger wardens on a 100 block radius.

Zombies

Will spawn holding a wide range of weapons, as well as shields.

Baby zombies can spawn riding already existing entities.

Some zombie variants have been added.

Wither Skeletons

Will spawn wearing golden gear.

Most of the wither skeletons found on the nether will spawn as piglin wither skeletons, wearing the same gear as piglins.

A reduced amount will be regular wither skeletons, equipped with gear from the overworld and, in some instances, bows enchanted with flame.

Rarely, they will spawn as illager wither skeletons.

Wither skeletons will drop withered bones instead of regular bones.