What is the pho "original" chile?

I'd go with Bird's Eyes, or as they're sometimes called, 'finger' chillies.
I'd define it as skinny, thin skin, lots of seeds - finger tend to be larger, green; bird's eye smaller, red.

The trouble with chillies is that they can all look a bit alike & trying to buy one very specific type is really not easy. Is it a jalapeño, is it a fresno or a serrano… what's the real difference? Can you tell just by looking, or even tasting?

You can easily tell the difference between a scotch bonnet & a jalapeño, of course, not only do they look completely different, they taste & smell completely different, but a lot of the others come in much finer distinctions.

Finger chillies & bird's eyes are close to being the same thing. They're not identical, but to the consumer it's really tough to tell them apart. Supermarkets tend to call the larger ones finger & the smaller ones bird's eye - they taste the same. See https://www.sainsburysmagazine.co.uk/lifestyle/food/a-guide-to-chillies for how the supermarkets very, very broadly differentiate.

BTW, I can tell the difference between a jalapeño & a fresno… but not if the ones the supermarket is selling this week look like something half-way between the two.

Late thought - cayenne looks like a 'finger' variety too; All I can think is they call them finger if they're green & cayenne if they're red. They never really tell you exactly what they are.