Optical Ethernet gains great success in varied optical domains. Ethernet while providing the elastic allocation of bandwidth per flow is currently restricted to being transported over fixed-rate fixed-grid optical channels. However, as data transmission and switching in optical networks is becoming more flexible, the current Ethernet to optical interface with fixed functions and fixed design face great challenges for applications in an elastic optical network. In addition, the coexisted Ethernet standards in optical networks also need a converter to transform low-capacity Ethernet signals to high-capacity Ethernet signals. In this paper, we propose a real-time Ethernet to software-defined programmable sliceable transponder based on network function programmable Ethernet to elastic optical networks (ETH-EON) interface, multiformat supported transmitter and architecture-on-demand (AoD)-based superchannel signal processor. The transponder parses the Ethernet traffic and then transports it over a number of optical carriers with the appropriate baud-rate, modulation format and associated spectral bandwidth. Software-defined reconfigurability of both electronic (i.e., FPGA) and optics (AoD) allow for a flexible transponder to offer high Ethernet to optical transport efficiency. Ethernet capacities from 40 Gbit/s to 2.88 Tbit/s can be provided by reconfiguring the modulation formats and the used optical carrier number. The modulation format adaptability also provides a tradeoff between transmission capacity and transmission distance, to optimize network performance. The designed software-defined sliceable elastic transponder aggregate multiple Ethernet traffic flows and send spectral-efficient signals through the elastic optical network with simple Ethernet protocols, avoiding using complex and expensive OTN technologies. Finally, we demonstrate the converter in elastic optical networks for several application scenarios.