The present study is focused on examining the flow-field dynamics of a flapping foil with a flexible aft tail as compared to a rigid configuration where tail flexibility is infinite. The flow around the oscillating body is governed by the incompressible Navier-Stokes equations. An in-house Fluid-Structure Interaction solver has been developed following a discrete forcing type Immersed Boundary Method coupled with an inextensible filament structural model. The flapping amplitude is considered as a bifurcation parameter, and as the bifurcation parameter is increased, the periodic wake transitions into the chaotic patterns. The periodic to chaotic transition happens through an intermittency route. However, the elliptic foil with flexible aft tail exhibits chaotic onsets much later compared to the foil with a rigid tail. Time series analysis techniques, such as frequency spectra and recurrence plots, have been used to establish the intermittency and the chaotic dynamics conclusively. © 2020 American Society of Mechanical Engineers (ASME). All rights reserved.