"Non" refers to old pets called the nontag, nonswirl, and nonjewel (from
July 2008). Those dogs were named as such for their lack of specific unique design that they were missing compared to other littermates. "MA" is short for mid-advent which specifically refer to a certain number of dogs released in the middle of the Christmas 2009 advent. These dogs are only the ones starting with the light colored PPS with the heart design on the back leg to the grey border collie with red collar
on this page.
Anyway, the nondogs and mid-advents were all very popular and became used as reference of value. So something that is worth "0.5 non" is worth 1/2 of any of the nondogs (ie, you would need two of those pets to be considered about fair for one nondog). Or for something that's worth 2-3 MAs, it's referring to something worth more than a mid-avent (so you need multiple MAs to be fair for that one pet) but not as much as a non. In recent years, with so many more pets and some older pets becoming more popular/difficult to find/etc., some other pets may be considered the value of =1 non or =1 MA, but are not actually a nondog or mid-advent pet, but they can be used as the value of one (for example, if someone said a certain pet is worth 1 MA, you could fairly trade with a pet that's not a mid-advent but considered worth 1 MA still).
As far as I know,
this guide is currently the most popularly referenced for values on older and popular pets. Do note that it is a user-made guide, though, and not an official guide or "rule" so just because someone doesn't use the guide or may disagree from it in some way doesn't mean they're trying to scam someone.
-fireflii