Horizontal transfer (HT) is central to the evolution of prokaryotic species. Selfish and mobile genetic elements, such as phages, plasmids, and transposons, are the primary vehicles for HT among prokaryotes. In multicellular eukaryotes, the prevalence and evolutionary significance of HT remain unclear. Here, we identified a set of DNA transposon families dubbed SPACE INVADERS (or SPIN) whose consensus sequences are approximately 96% identical over their entire length (2.9 kb) in the genomes of murine rodents (rat/mouse), bushbaby (prosimian primate), little brown bat (laurasiatherian), tenrec (afrotherian), opossum (marsupial), and two non-mammalian tetrapods (anole lizard and African clawed frog). In contrast, SPIN elements were undetectable in other species represented in the sequence databases, including 19 other mammals with draft whole-genome assemblies. This patchy distribution, coupled with the extreme level of SPIN identity in widely divergent tetrapods and the overall lack of selective constraint acting on these elements, is incompatible with vertical inheritance, but strongly indicative of multiple horizontal introductions. We show that these germline infiltrations likely occurred around the same evolutionary time (15-46 mya) and spawned some of the largest bursts of DNA transposon activity ever recorded in any species lineage (nearly 100,000 SPIN copies per haploid genome in tenrec). The process also led to the emergence of a new gene in the murine lineage derived from a SPIN transposase. In summary, HT of DNA transposons has contributed significantly to shaping and diversifying the genomes of multiple mammalian and tetrapod species.