It sounds like if you have say animal A, B and C, then if you have for example
A with frequency 1
B with frequency 10
C with frequency 5
the total "amount" is 16, thus the actual frequency of animal A is 1 in 16 (or 6.25%). Or for B it is 10 in 16 (or 62.5%).
If you increase C to 10, not only do you increase it's relative probability to spawn, you decrease the others (A would then have a probability of 1 in 21; 4.0%).
Adding animal D with a probability of 5 results in a probability of 1 in 26 for A (3.85%).
So to get the actual probability for a specific animal to spawn you have to take the sum of all the frequency-numbers and divide your animal frequency with that. The problem is that whenever you make a single change you essentially modify the probability of all the other animals as well
The easiest is probably to put it all in a spreadsheet with automatic calculation of probabilities and set everything to 10, that way you have leeway to modify numbers both up and down.
This may all seem unnecessarily complicated, but I can't honestly find any better method to be able to change multiple probabilities without going under or over 100%.