While you can choose your order after the first boss, this was my 2nd boss fight. Yoshi confonts Kamek and Baby Bowser and they decide to create a new monster.
And it's a piranha plant monster (how original) made of acorns.
Since this type of boss has been in previous Yoshi games, you'll know mostly what to do here. Try not to get hit while he throws things at you.
And wait for these orbs to appear. Make a spike ball land on him, and he'll suffer greatly for it. Not my favorite boss in the game, but a decent one.
After he's defeated you get the red Dream Gem back.
Only three more to find.