A while back, monsters (aside from mythic-rarity monsters which this does not apply to) were given "global bonuses" that help you out in other areas of the game. Please note that these bonuses do stack. Global bonus percentages can be raised (upgraded) by transforming the star rank of any monster. The global bonus for each monster is listed in the top right on when veiwing an individual monster's stats (found in your "monsters" section in the bottom right of the game's home screen). The global bonus a monster provides is always for the same color type as the monster itself, and either affects all monsters of that same color, or buildings of that same color. The boosts given are a percentage increase of either health or attack.
Epic monsters require fewer monsters to transform, and also grant a higher global bonus percentage increase with each transform. This scales downward with rarity, meaning the more common a monster is, the more copies required to transform it, and also the less it's global bonus percentage increases. In addition to this, epic monsters' global bonuses can go all the way up to a 100% increase, while rare monsters can only reach a max of 50% and common a mere 25%.
The addition of this feature has changed the meta of the game entirely, giving purpose to many monsters previously untouched, as well as allowing nearly all monsters to be feasible additions to teams used in PvP/PvE. That being said, knowing which monsters do what is now a key part of figuring out what's worth leveling up to transform, but it can be tedious to constantly go back and check your monsters page to see which monsters add which boost to what. I am providing an organized list below for your convenience. Please note that the letter in parenthesis next to a monster's name denotes its rarity.
Increases Monster Health
Eelectricus - (E)
Hydra - (E)
Naga - (E)
Anubis - (R)
Plasmire - (R)
Barf - (C)
Increases Building Health
Rompus - (E)
Cragg - (E)
Norman - (R)
Hach - (C)
Maximus - (C)
Increases Monster Attack
Bubbles - (E)
Dante - (E)
Ginsu - (R)
Salty - (R)
Kudzu - (C)
Mogu - (C)
Increases Building Attack
Mutt - (E)
Tosca - (R)
Pockitz - (R)
Grimm - (C)
Don't forget that the color of the monster is the same color your global boost affects, so if you want to focus on upgrading blue monsters or buildings, focus on transforming blue monsters only, and vise versa for the other colors. It can also be wise to focus on rarer monsters for a larger global bonus increase each transformation. Feel free to ask any questions below, and happy smashing!