Zenfone 4, 5, and 6 were released last year and the number were based on their screen size. Asus also quietly released Asus Zenfone 5 lite and Zenfone 4.5 in certain markets. Zenfone 2 looks to be their model for 2015 but Asus tends to release variants of it as well. Yeah, it can be confusing as I am still figuring out why Huawei named their phablet with near 6-inch screen the Ascend Mate 7.
Check Amazon for availability. Asus started off making smartphone/tablet hybrids with the Padfone series. Then they went 7-inches with the Fonepad series. But in fairness, their phones are quite solidly built like their Taiwanese neighbors, HTC. When I tried comparing the Xiaomi Redmi 1S vs Asus Zenfone 5, the latter had the better display and camera. I would go far to say the Zenfone (5/6) series is a better deal than the Moto G's and the $100 Zenfone 4 is a better deal than the Moto E.