How to install specific version of bitcoind
Source code for Bitcoin Core, v0.12.1 can be downloaded directly from GitHub, via https://github.com/bitcoin/bitcoin/releases/tag/v0.12.1

Binaries are available at:  https://bitcoin.org/bin/bitcoin-core-0.12.1/

Be aware, v0.12 has known vulnerabilities, and should not be used in new work.  https://bitcoin.org/en/release/v0.13.0 shows some of the vulnerabilities that were fixed in v0.13.