I’d say it kind of depends on what type of pony they are. The earth ponies are probably the heaviest, while pegasi are probably the lightest. My guesses would be:

Earth: 250-350 pounds. This is due to the fact that real life ponies typically weigh about 450 or so, and the fact that the show’s characters have some differences in anatomy from real ponies. They also likely weigh more than the other races because of their earth pony strength and durability.

Unicorn: I’d guess around 175-275. They seem like they’d kind of be in the middle of the other two when it comes to weight. They don’t have any reason to be heavier, nor do they have a reason to be lighter.

Pegasus: Maybe 125-225. They need to be lighter than the other races so they can carry themselves in the air. Wings aren’t typically powerful enough to carry 400+ pound creatures, after all.

Of course, there are deviations to this. Bulk Biceps could maybe weigh as much as 300, and Big Mac could probably weigh up to 500 or so.