Okay so, first things first, as jungler Pantheon you will be relying on 4 key things and ill go into depth about each one, i will also go over the CONS of this jungler

1. Damage: The damage that pantheon does is critical to how good he can be as a jungler, as most of his abilities use AD you will be ofcourse building AD weaponry.

2. Speed: Another key thing to note about jungle pantheon is how quickly you can get to and from lanes to help gank so knowing where to gank and when as well as how quickly you can clear your jungles with his mana pool is essential.

3. Ulting: For jungle pantheon his Ult is his secret weapon as it enables you to effectivley be in any lane and bypass ANY wards they have because you jump right over them.

and 4. Teamwork: Jungle Pantheon relys heavily on team work as his ult kind of declares hes coming to the lane, so pinging and informing your team whats about to go down is essential, just be aware of what your team is doing.

Now for Jungling pantheon as i mentioned before you will be relying heavily on his damage output, so the first item i would rush is the Spirit of the Elder Lizard as it gives you alot of extra damage (due to the two blades you need for it) as well as helping you clear the jungle faster and giving you a tiny bit of burn damage for possibly snagging a fleeing enemy. (very tiny chance but hey it can happen)

Next i would grab another blade for making into a Brutalizer as well as a set of standard boots, as this will enable you to chase better (or run)as well as giving you a tiny bit more damage output.

Then grab your boots of mobility and the brutalizer, as the boots will enable you to, say sprint to top lane for a quick gank then ult down to mid lane for another, then sprint to bot for another!
the BoM gives him incredible speed and lane pressuring powers.

After this im assuming you should have a few kills or atleast a ton of assist so you should have gold, grabbing damage items such as a BF sword is nice or even finishing off the Black cleaver (built from the brutalizer) as finishing the black cleaver will give you a bit more health due to the ruby crystal.

now if you seem to be launching into team fights where you and two team members die but you win the fight, you can build a Guardian Angel (i build one EVERY game) it gives pantheon obnoxious power as he deals incredible damage dies then gets back up to do it all again.

After this its completely situational to what there team is doing.

Just remember Build damage items FIRST armour and such LAST

Pantheon is a unique beast as he has incredible AD damage potential from his abilities.

Pantheons best combo is initiate on your victim with W to stun, then IMMEDIATLY follow that with an E as you should land 2 or all of your stabs onto them to do loads of damage, then as they try to run throw your Q at them, now usually this combined with a few hits from your team mates will take anyone down.

Later on you will be using the same combo just this time initiate with R, land and use W, then E then Q, same principle, burst. them. down.

Now for the biggest part of jungle pantheon, i realized that Zac can jump from out of conventional areas and land in lane for a gank and it works really well, so why cant panth? his jump is much longer and does WAY more damage. Simply put, he can!

What you want to do is ping to your team who your going for, say you go bot lane and want to pounce a Caitlyn, ping on her then ult JUST BEHIND HER as she will no doubt see it and try to run, but usually as you land she is still in range, so initiate your combo and kill kill kill!

Failing this, or if you feel the lane is a tad dangerous or you are low on health, you can ult into a bush at bot or top lane as they WILL NOT see the circle of your ult(unless the brush is warded) so leap into a bush and jump out from behind them for even more suprising and confusion!

Now for late game as pantheon isnt nearly as good at initiating as say Sejuani its ideal to wait around near your team in a brush and when a fight kicks off dive ontop of the team and fight away as your ult does a ton of damage for anyone in it.

but first and foremost as pantheon jungler FOCUS THE ATTACK DAMAGE CARRY. ALWAYS. NO EXCEPTION.

The reason for this is pantheons W enables him to land on the adc and burst her down with the previously mentioned burst combo, thus nullifying the teams long range damage capabilities, you will probably die doing this but thats what the GA is for, so you can get right back up and land on the mid laner and do the same (provided your team isnt completely hopeless)

Now like with anything this kinda plays will take practice, but normally if you're playing as pantheon and you get two kills early game he is near unstoppable in jungle as he can jump to lanes for ganks and the BoM enable him to be friggin everywhere!

Also if you feel brave as pantheon you can counter jungle by sneaking into the enemy bushes and pouncing when the enemy least expect it as you can stun and secure the kill from them getting away which makes your early game insanely hard to stop.