Tokamak plasmas present many interesting nonlinear aspects. The fishbone instability is a good example of this kind of irregular and complex behavior. A simple heuristic non- linear model has been developed to study the fishbone repetition cycle. The model consists of two coupled non-linear differential equations, which describe the evolution of the mode amplitude and the resonant fast ion density as a function of time. This model predicts two forms of fishbones, i.e. short repetitive bursts as well as continuous oscillations. An extended model includes the slowing down of fast ions, two types of loss mechanisms (particle diffusion and ergodization of the fast ion orbits) and a periodic forcing term due to other MHD events, such as ELM's. This refined model allows more complex solutions which qualitatively reflect the irregular behavior sometimes observed in JET experimental data.