On a scale of 1 to 10, how hot is Arcanine?

Well, almost all Fire Pokémon are explicitly described by the Pokédex as having a very high body temperature or extremely hot fire attacks.  Even if it doesn’t give us an exact temperature (and we shouldn’t necessarily believe these anyway; the 10000 ºC quoted for Magcargo’s resting body temperature is hot enough to vaporise steel), it likes to wax lyrical about the intensity of the fire produced by many Pokémon, telling us in Emboar’s case, for instance, that “a flaring beard of fire is proof that it is fired up,” and “it can throw a fire punch by setting its fists on fire with its fiery chin,” and “fiery fires fire flaring fire fiery flares fire” (okay I made that last one up but you get the idea).  

Growlithe and Arcanine are just about the only Fire Pokémon whose Pokédex entries contain no reference to their fire abilities at all, focusing instead on their loyalty, speed, and regal appearance.  Also, while Arcanine’s special attack is high, it’s not that high; it’s actually about average for a fully evolved Fire Pokémon, and two of Arcanine’s three possible abilities (Flash Fire, Intimidate and Justified) are not Fire-related.  So I think it’s likely that Arcanine is actually on the low side by the standards of Fire Pokémon – maybe a 3 or 4, with Growlithe likely being 1.  Of course, if you were to compare them to all Pokémon (of which Fire-types comprise only 8% of the known species) you’d probably still be looking at a 9.