This huge threat also hits hard and is difficult to block, since only legendary creatures can be assigned to block it in combat
Granted, Bitter Triumph is card parity, while this is card disadvantage, and haste is a big part of what puts Bitter Reunion over, but making Treasures is a way for Reanimator decks to hard-cast their big creatures and is just a generally powerful thing to do
Elvish Mystic and Llanowar Elves might also see a lot of play, and are that highly lucrative Elf creature type, but they can only produce green mana
G1 My opponent starts with a Duress, grabs my Mox Diamond to prevent a first turn Sylvan Library
A quick Magic Online search shows that there are nearly 150 rare Dragons in Magic but only about 40 rare Cats
You can find those rules inside the box that came with your starter deck