Having caught 500-1000 Pokémon isn’t all that much, take a look at your other Pokémon and see how many you have caught and have not seen a hundo… it is all down to how lucky you are.
You have about a 1 in 4000 chance (see whate @PokeprofJulian wrote) to see a hundo but that means that for:
your 1st caught your chance is 1/4000, your 2nd caught has 1/4000 chance,… your 100th has 1/4000 chance, your 4 000th caught still has that same chance.
If you have average luck you will see a hundo in those 4 000 catches, if you are really lucky you will see more then 1 in 4 000 catches, if you are unlucky you will see none not even after 10 000 catches.
I have caught more Meltans then you need to complete the Steel Platininum Medal but still have no Hundo, I have 1 98% and a handfull of 96% but still this is what you can expect.