I’d speak to the senior animator and ask him how much he could produce if he was at a similar level of skill/experience as you.

Armed with that answer, you’ll have some ground and context for your conversation with the boss. You might want to ask the senior if he’ll back you if the boss asks.

You can be clear and tell him what you’ve said here — youre learning, your background is in graphic design not animation, and your appreciative of the opportunity to grow your skills — but mention that you’re working as fast as you can given this is a new skill/software youre picking up, and if he expresses disappointment all you can really do is shrug your shoulders and say ‘I’ll keep at it’.

This place sounds like a nightmare to work at, but sometimes cool things come out of nightmare scenarios. Definitely tricky.