Translation:These snakes are striped, but those are spotted.
'those ones' is not standard (formal) English. Standard is just 'those'. In other words, 'those' can be a pronoun as well as an adjective/determiner.
For example, compare:
1. Those ones over there are on sale.
2. Those over there are on sale.
You will hear 1 from time to time, but 2 is what is taught to and used by most speakers most of the time.
Not sure about that. "Those ones" is totally standard English, though colloquial.
why is pedig used here? Isn't pedig used for opposition or contradiction and exclusion. I mean english sentence with but seems a bit clunky to me, that is not a way to use but (or but is not the word i would put here)
I think 'whereas' works well here as a translation for 'pedig'. In any case, there is certainly nothing wrong with or weird about 'pedig' in the Hungarian sentence!