There are a few bosses in terraria and they can be summoned,spawn,or even spawn during an other boss fight!
The Eye of Cuthulu
And the Eater of Worlds A.K.A EoW
The Wall of Flesh A.K.A WoF
The Twins(see The Eye of Cuthulu)
The Destroyer (see EoW)
And the Skeletron Prime (see skeletron)