Ben, the original creator of PsyPets, created and ran PsyPets for several years until he put his code on GitHub (link) and eventually ceased updating the original game. To continue the ideas and fun of PsyPets, Amanda 'Onyx' Kroft and Matt 'Grimm' Oberlander created TerrePets.
Psypets was on PHP, but when remaking it I went with what language I was familiar with and what I thought would work well for the site.
— Onyx (2016). TerrePets forum (post link).
The main chunk of TerrePets's code can be found on GitHub (link) but this hasn't been updated as of 2014. Any secret information or formulas along with graphics are not included.
Continuing the ideas of PsyPets, the pets in TerrePets have needs which follow Maslow's hierarchy of needs. Pets need to feel loved, safe, and esteemed in order to be productive. And in order to feel loved, safe, and esteemed they need to not be hungry or tired.