Answer: Taenia species tapeworm segment.
The segment shown is notable for the fact that it is still moving (!) as well as the fact that the proglottids are longer than they are wide. The latter feature, in addition to the large size of the worm, supports an identification of Taenia species (as compared to Diphyllobothrium spp., Dipylidium caninum, and Hymenolepis spp. Unfortunately, we are not given any additional morphologic features that would allow us to determine which Taenia species is shown (such as the uterine branching pattern or scolex). However, Florida Fan and other readers correctly note, that the history of eating undercooked beef supports the supposition that this is T. saginata. In comparison, Taenia solium and Taenia asciatica are acquired by ingesting undercooked pig (muscle, or in the case of T. asciatica, liver and possibly other organs).