Transgenesis is an important and often irreplaceable method to study numerous processes of animal life. To create animal transgenic lines, it is necessary to have a suitable model organism that has necessary traits for efficient and affordable transgenesis. The concise review characterizes the existing model organisms of different taxa for which an efficient transgenesis protocol has been developed. Special attention is paid to flatworms and, in particular, Macrostomum lignano as a promising model organism for studying aging, regeneration, and carcinogenesis.