AIN is a compressed archive format, produced by a DOS shareware program of the same name. It was developed by Transas Marine Ltd.
Self-extracting AIN archives?
The AIN software was distributed in what seems to be a "self-extracting AIN archive", and the file command agrees that that is a thing. But we haven't seen any evidence of a publicly-available program or feature for creating such files.
The documentation says "AIN version 2.21 archive format is incompatible with that of AIN version 2.0."
At least some .AIN files start with the byte sequence (hex)
21 ?? 00 00 00 00 00 00.
AINEXE-compressed EXE files most likely have ASCII "
AIN" at offset 32.