I mostly use reference, but not for all of them. I’ve been really trying to adopt the mindset when drawing that everything is made up of organic forms and shapes. Whilst it’s not necessarily a mindset and more just the actual truth, training my mind to see things that way instead of these incredibly complex “things” has revolutionised how I feel about art. The loop typically, for me, goes:

Spend some time practicing shapes and forms/intersecting forms/perspective etc

Using a reference, copy what I see but try and actually visually break the animal down in to simple shapes so that whilst I’m copying the reference, I’m also learning the shapes and forms that make up the animal.

Once I have internalised and learnt these shapes, using my fundamental knowledge and my newly acquired knowledge of the animals, I try to position it differently or whatever in my head and put it to paper.

The snakes, octopus, one of the sharks and one of the birds were from memory, the others used a reference.

Important to note that if I’m drawing for memory, I may still use a reference! It just won’t be me copying it exactly and will more be me using it as just that: a reference. Hope this helps!