Will add my own screencaps latter, for now using the ones modders made.

Pupulate your world with animals, more animals than you've ever seen spawned at a given time.

Uizex1's "Live Blocks" are amusing to watch, especially when they're swiming. http://www.planetminecraft.com/mod/live-blocks/


LostKingpin's "Loafcats" are as tamable as wolves: http://www.planetminecraft.com/mod/loafcat-ver081/


Also by LostKingpin is "Fishy," with the large fish that swim in & on the water. http://www.planetminecraft.com/mod/165-fishy-ver06/


Steelroad's "Squirrel Mobs," you'll never catch them. http://www.planetminecraft.com/mod/squirrel-mobs-for-166/


DrZhark's "Mo Creatures: http://www.minecraftforum.net/topic/81771-v173-mo-creatures-v2122-with-kitties-mice-and-rats/
Passive:
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*Kitties (compatible with Loafcats).
*Mice.
*Rats.
*Deer.
*Fish.
*Ducks.
*Rabbits.
*Birds.
*Foxes.
*Horses.
*Dolphins.
*Unicorns?

Probably aggressive:
-----------------------------
*Sharks.
*Werewolves.
*2 types of Bears.
*A new wolf.
*2 types of Wraiths.
*3 Types of Ogres.
*Boars.
*Piranha.
*Large wildcats.


Cocoavalley's "Baby Animals" will not replace default mobs. http://www.minecraftforum.net/topic/190053-173-baby-animals-v175-mobml/

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Tired of only having three kinds of trees? Want to add without replacing the defaults?

Nandonalt's tree mod only works on newly spawned maps, & some these trees are easily twice as tall as default trees. Also check out his or her coral reef mod. http://www.minecraftforum.net/topic/113054-166-173nandonalts-mods-coraltreesa-lot-more/#Trees Same link includes Nanodalt's Chocolate Mod, which comes with a cocoa tree. To access the mod, open the spoiler for the mod, scroll down to the bottom & look for the word "link" highlighted in blue. Coacoa tree will spawn on already existing maps.
*Palms.
*Banana trees.
*Sequoias.
*Sakura.
*Apple.
*Eucalyptus.
*Vines.


Crunchy Cat's "Nether Trees" add color & light. http://www.planetminecraft.com/mod/nether-trees/


Sandrohobbins' "Golden Apples & Golden Trees" look like fall aspens. Youcan use them for gold or for pure decoration. http://www.planetminecraft.com/mod/golden-apples-and-golden-trees/