Background: In an initial pilot study, we investigated an animal-assisted treatment (AAT) procedure with sheep as an adjunct to treatment as usual (TAU+AAT) in psychiatric inpatients with substance use disorder (SUD). Over time, this TAU+AAT intervention significantly reduced negative emotions and improved positive emotions, mindfulness, and self-efficacy expectancy compared to TAU. In the current study, we aimed to replicate these results and extend the investigation to another group of inpatients with difficulties in emotion regulation, namely borderline personality disorder (BPD). Methods: A single-session AAT procedure with sheep in a group setting as an adjunct to treatment as usual (TAU+AAT) was examined in an RCT compared to TAU. A total of 29 psychiatric inpatients with SUD and 31 with BPD were examined (PRE vs. POST) using questionnaires on variables that included positive and negative emotions, mindfulness, and self-efficacy expectations. Results: In the SUD sample, significant effects between PRE and POST, with large effect sizes in all four outcomes, emerged for the TAU+AAT group, in contrast to TAU. In the BPD sample, similar time (PRE vs. POST) and group (TAU+AAT vs. TAU) effects were achieved for all outcomes. Conclusions: Repeatedly, this TAU+AAT intervention, which involved a single session with sheep, improved in all outcomes. Sheep seem to be suitable for AAT with a focus on mindfulness.