Is Mareep the new Grimer?
The drop rate for Mareep seems pretty low and it's final evolution might be the last dex entry for alot of people. Which sucks because Amarphos is such a beast.
Answers
As for my personal experience Mareep is like 10 times or more rare than Grimer, I have at this moment 6 Muks, 2 of them caught in the wild on my town, and Grimer/Koffing is not that rare here, I would say I can get 1 of each every day, they have a frequent spawn rate, not as a normal mon of course, they are rare, but you know what I mean.
To make a comparision on the last week since gen2 has been released I have only caught and seen one Mareep and one Larvitar, but got like 9 koffing and 5/6 grimer.
It depends a lot on the biome you have, for me some gen1 mon were ultra rare and when I say that to other friends they laugh at me saying those mon are the most trash and they get 100 every week, some examples are Mankey, Machop, Geodude, Growlithe, Ponyta...It took me a LOT and the help of hatches to get those and their evolution on the pokédex, it can take a month here and only see 1 geodude for example.
Medium sized town, 90% of spawns pidgey and rattata (now sentret and spinarak most).
The town has 2 parts, one close to a river where most of spawns are clefairy, oddish, bellsprout, hoppip, bulbasaur, chikorita, with some magikarp and slowpoke.
The other part is the normal stuff, bug type and normal type mon (now completely full of murkrows) with some rare now and then, like Porygon, Chansey and some ocasional Snorlax, is on this area where I see normally Grimer and Koffing, this part of the town has very few stops (3).
Of course I have to say that finding them has become harder because of the new tracking system, that keeps adressing me to a stop that is a km away while I have like 50 or more spawns less than 50m from me...