The Thistlehead is a weird one.
The Shadow Figure corrupted Ken and changed his DNA in order to change him to resemble that of another creature.
The English name of this monster I will not tell (I actually haven't named it yet, lol) but the scientific name is Dracovirum lupus.  Meaning Dragon headed, wolf.  However, contrary to it's name the creature is quite mammal like.  You could compare the pure species to a Dimetrodon, which is a non-mammalian synapsid that came before mammals and the dinosaurs.

The pure species is a creature native to The Underworld and can be found in both main universes version of the underworld.  They would be semi quadrupedal, meaning they are predominantly bipedal but can walk an all fours.  However, because of their long claws, they use the back of their hands for walking and tuck their fingers away.  They have cushiony pads on the back's of their hands to allow for this.  This style of walking is similar to how we think Spinosaurus lived.

The spikes on it's back are not actually bone but are composed of keratin, the same material that makes hair, fingernails, deer antlers and rhino horns.

The version we see of the Thistlehead in Wairangi actually doesn't have all of it's features yet and a few new additions will grow in as it matures.

Using a form of Dark Magic, the Shadow Figure was able to mutate Ken into this human x monster hybrid, but she did so as the species is easy to control.  The creature makes a good pet or servant as it can manipulate objects in it's hands, allowing for a lesser version of precision grip. 

​Remember that some of the exact features and proportions may not be consistent with the pure species, because of the human base the new features had to fit around it.  Not everything is compatible so many features will be missing or replaced with an equivalent.  A good example of this is the head, Ken's skull had to drastically change and this skin warped, peeled and stretched as the new skull grew.  As this was happening, the cells where rapidly dividing in order to heal the wounds.  This resulted in a scared, bruised and torn flesh all over the head.  A pure version of the creature would have normal, smooth skin as it would not have undergone this process.