New ternary rare-earth indides RECoIn (RE = Gd–Tm, Lu) have been synthesized by arc-melting the elements under argon and subsequent annealing at 870 K for 1200 h. Single-crystal X-ray diffraction revealed ErCoIn to crystallize in a new structure type in oP100, space group Pbam and Wyckoff sequence hgda with a = 23.203(5), b = 28.399(5), c = 3.5306(6) Å. The crystal structures of RECoIn (RE = Tb, Ho, Er and Tm) were determined from single crystal and powder X-ray diffraction data and further investigated by DFT methods. The compounds belong to a large family of ternary rare-earth indides with intergrowth of the AlB- and CsCl-type related slabs. In the ErCoIn structure, four types of fragments REIn and RET of CsCl-type, as well as RET and REIn of AlB-type, are present simultaneously. A simple Python tool was developed to determine the coordination number for each crystallographic site with various methods and tested on the complex structure of RECoIn.