teacup is a general term for a small freshwater ray, but 90% of the time the common teacup ray is protomygoron hystrix, (not sure about the spelling, too lazy to look it up right now). As far as a ray in a 55gallon tank is concerned, no ray that you want to deal with will fit in that size tank, and I wouldn't keep any with a puffer either, they tend to be very nippy. The one ray i've kept that would be a potential candidate for a smaller tank would be a lesser electric stingray, They max out about 12" including the tail and are very innactive, but feeding is very difficult. Mine only accepted annellid worms, (mostly spaghetti worms and bristles that I pulled out of reef tanks I serviced). In the end I lost her after about 10 months. Tankmates would also be very tough to come by.